That One Boss / Arknights

Arknights boasts a wide variety of boss encounters designed to challenge players, and while difficulty is often justified for boss battles, there are a couple among them that seem designed to make Doctors pull their hair out.

Main Story

• Skullshatterer, whose appearance is often a definitive sign of the game taking its easy mode gloves off. Packing extremely high HP and a huge ATK stat, they deal really high ranged AoE damage with their grenade launcher on top of temporarily cleaving your operators' defense in half, opening them up for more damage from themselves or other physical enemies. Not only are they capable of both ranged and melee strikes, their damage output is far worse when they're blocked, since their melee attacks hit a lot harder than their grenades. Once Skullshatterer gets below half health, they'll gain a 50% ATK boost. If you can't kill them quickly at this point, they can wreck your entire defensive line very fast with their AOE damage and devastating melee strikes. While their first boss fight is manageable, the 2nd boss fight against them can be rage inducing due to their inflated stats. While their second stage gives you defense boost tiles, it's pretty pointless once they turn red. Even top tier Defenders well above the recommended level and placed on the defense tile will be cleaved in half very quickly if said Defender isn't backed up by several Medics.
• Faust. He's a very (read: insanely) hard-hitting, global-range Sniper enemy who cannot be harmed for the entire duration of his first phase due to his stealth, which lasts for two and a half minutes. During this time he will casually snipe your operators from across the map with disturbingly powerful shots from his crossbow - and every once in a while, he will launch a Critical Hit that deals upwards of 2k physical damage, enough to instakill just about any squishy and punch holes in whatever survives. On top of the constant damage they're already taking from nearby enemies, even sturdy defensive lineups can easily crumble thanks to his interference. To make things even more annoying, he will also periodically summon invincible ballista turrets that ramp things up to eleven, necessitating that your deployment order must be on point and your lineup be beefy enough to tank all of that - sending in anyone after the operator who is supposed to tank it, is basically sending them to death since Faust will target them instead of your tank, so it is important that the healer who is banking on them is deployed first. Oh, and he also can't be blocked, making it impossible to stall his advance once he gets moving, especially since he also high HP. Now try escorting your squishy DPS units through all that mess because you need them to kill Faust.
  • Faust stars as the main boss of Operation Blade's permanent map, which not only lets him take advantage of all the global contracts, but also gives him two particular contracts to specifically buff him further. Sharp Arrow buffs his stats and lets him spam ballista turrets that have heavily boosted damage to pressure the whole map simultaneously, while Snake's Eye buffs his already high ATK even more (but buffs his defenses less than the alternative) while letting him spam critical hits that deal One-Hit Kill levels of damage.
  • However, while the ballistas can be dealt with with very good healing and the crits can be soaked if you can count to five (seven if Snake's Eye wasn't taken) and use fast-redeploy units to take the hits, the real threat is the ridiculous HP and DEF buffs it gives Faust. Even if you can completely neutralize his attacks, you will still have major problems getting through his absurdly high HP pool before he struts into the blue box, especially if combined with other contracts; and since you cannot block him, this essentially forces you to pass a very tight DPS check.
• FrostNova is notoriously difficult, mostly because of her very annoying arsenal of skills rather than her raw stats.
  • While FrostNova's power comes from her skills, she's still an Arts caster with respectable stats, meaning she will still be dealing heavy ranged damage. She can also periodically fire a radial nova that deals Arts damage to everything around her while severely lowering their attack speed, making killing her all the more difficult.
  • From the get-go, the most aggravating thing she can do to you is fouling certain tiles with magical ice, which instantly eliminates any operator standing on them regardless of health, on top of making them permanently off-limits. This can severely screw with your unit placement, especially during Auto Mode, considering its already-wonky AI ^note  Although enemy RNG is fixed in Auto, it can still happen . Worse still is that whichever tiles she denies are mostly random, unless you can coax her out by placing some operators in her way as bait, or by bursting her down quickly enough to transition her into her second phase, which has an entirely different skill set. In her Chapter 6 encounter, she can freeze even more tiles at once once she Turns Red, up to 4 tiles at once in H6-4.
  • Speaking of second phase. FrostNova generally takes more effort to put down than other Reunion bosses due to her reviving herself upon her first defeat, which also makes her into a walking powerhouse that can burst down your operators very quickly with her buffed attack. It's possible to cancel out this revival with careful application of Shift operators with upgraded skills to jostle her before this ability kicks in, but it's impractical to do and doesn't always guarantee success. In her rematch, it's even worse, since she gets a sizable chunk of invulnerability after reviving.
  • Later on, during the Chapter 6 encounter, all of her attacks can now inflict a freeze that completely immobilizes your operators if she can hit them two consecutive times, and she can keep this up permanently on whoever she's targeting. In addition, not only does her radial nova cover a huge area after she revives, it now deals double damage to frozen operators, allowing FrostNova to deal a disturbing amount of burst damage to a huge number of operators at once. And that's with the ice crystals and ice-based enemies already present on the map. This is one of the few places where Wide-Range Medics and their debuff duration reduction really shine.
  • That being said, compared to the ridiculous situation that is the prevalent Cold damage of Chapter 6, FrostNova seems like a cakewalk by comparison, as she's almost always encountered separately from the main bulk of her army, who are arguably the actual threats due to their sheer numbers and the extra damage they deal to frozen operators. Short of H6-4, FrostNova can be very quickly dealt with by a decent damage dealer like Exusiai, and some clever placement of stall tanks to weather out her invulnerability period. To make it a little bit more tangible, stage 6-16 is the first boss stage where most guides just tell you to bring a 6-star operator.
• Patriot. Mere hours after his debut in the Chapter 7, he was already being hailed as the toughest boss in the entire game, with extremely high stats and number of skills to make any Doctor pull their hair out in rage. Although Patriot has been one of the most infamous victims of Power Creep which makes him far easier than he was before, a newer player without an arsenal of Game Breakers under their belt will quickly find out why he was so feared in his heyday.
  • Passively, Patriot activates the special skills of all Guerrilla enemies on the map, which grants them bonus stats and occasionally new attacks. He also grants every enemy a +20% ATK buff and +200 DEF just by existing - and yes, this includes himself.
  • He enters the map in a defensive stance which grants him tripled DEF and doubled RES (which is 2100 DEF and 90% RES if you factor in his buffs). In this state, nearly everything deals Scratch Damage to him, leaving true damage like Amiya's S3 or a lot of stacked DEF/RES debuffs the only way to actually hurt the guy. The stage does give you true-damaging land mines to use against him, but you only have 15, and other enemies may detonate them (of note are the swarm of jetpacks that drop in partway in the fight and walk through the area you fight Patriot in, detonating most of the mines you plant there). Worse yet, he has an active taunt that makes all nearby Operators target him instead of, say, the horde of lethal and super-buffed enemies rushing at you (As an extra layer of 'screw you', this taunt has higher priority than that of Guerilla Shielders, making it even harder to dispose of those already extremely strong enemies). And the cherry on top is that he also has an effective 70% ATK bonus in this state, along with his attacks being a combo that strikes 4 times consecutively.^note Even at his weakest, that's an effective 10,880 physical damage in a single attack cycle. Nothing short of invulnerability will save anyone he decides to attack in this state.^note  although it is not impossible, you'll need massively buffed defenders and possibly some ATK debuffs. For example, Nian with her S2 while under the buff effects of Aak's S2 and Shining's S2 with at least some skills on mastery might barely have enough bulk just to survive
  • If you bring him down, you're not out of the park yet - taking a leaf out of his daughter Frostnova's book, he will become invincible and slowly revive himself, while gaining an aura that deals a disturbing amount of true damage over time to every Operator near him. And once he revives - good luck, because while he's not Nigh-Invulnerable anymore nor can he Draw Aggro, he still gets substantial attack and defense buffs, while retaining his burning aura. Oh, and he can No-Sell stuns now.
  • In his second phase, Patriot's attacks are now able to hit both melee and ranged tiles, letting him kill off your damage dealers just as easily as he does your tanks. He also gains a lethal ability used every 15-20 seconds where he throws his spear at the furthest ranged operator, dealing enough damage to instantly kill them regardless of who they are^note ...except Amiya, whose Chimera skill actually assures that she will survive the spear with a sliver of health. You could also use a Beeswax who is significantly leveled and has her defensive talent up, or someone like Shamare to reduce his ATK while simultaneously using other means to buff the target's DEF - otherwise, your only other option is to rely on operators with a chance to dodge and pray to RNG..
  • If that wasn't enough already, Patriot also returns in H7-4. Not only does he have even higher stats, much stronger backup, and a legion of Guerilla Shieldguard Leaders to protect him, this state doesn't give you any Interference Mines to help. Good luck!
• Mephisto in his mutant state as "The Singer" can be a real hair-puller, mostly because of the poison cloud he releases every time he loses 20% of his max HP. This cloud debuffs every operator present on the map and deals a decent amount of true damage per second for its duration, but the problem arises with the fact that if he releases another cloud while one is already active, the duration resets and the damage will stack. If you damage Mephisto too quickly, the sheer damage the stacking poison will do is often enough to cause a Total Party Kill unless you have several healers working on overdrive. When you take out his first health bar and he enters his second phase, the poison cloud will be released at 15% HP intervals instead, making it even more liable to stack to high heaven - while it's possible to reduce his HP during his phase transition to make him enter phase 2 with less health (therefore making him unable to produce as many poison clouds), good luck doing that when his hibernation inflates his defenses to high heaven and gives him regeneration. While Mephisto himself isn't too threatening, he can also heal his allies and empower the ATK and ASPD of his Possessed flunkies as long as he's alive - while this goes on standby during his hibernation, both the buffs and his healing get even stronger after he Turns Red. This leaves you with a difficult Morton's Fork scenario - do you kill Mephisto quick to get rid of his team support and try to survive the poison, or do you slowly chip him down and contend with his army of juiced-up zombies?
  • It should also be noted that in order to unlock and view EG-3, you have to let Mephisto enter the blue box. This does not sound hard in of itself, but bear in mind that as a boss, he will deduct 2 of your 3 life points, which means that you cannot let any other enemies through, which is already unpleasant due to their souped up stats.
• Talulah is extremely dangerous as expected of the Big Bad and Reunion's leader.
  • While her ranged attacks only deal 40% of her ATK as Arts damage, she deals true damage on her melee attacks, on a significant minimum attack stat of 1500. Defense and Resistance are both useless here, so you best hope you never let her get close enough to strike lest she instantly cut down your Defenders like they aren't there.
  • She will periodically use Stellar Corona, which deals very heavy Arts damage in a wide radius around her. The massive radius of 2.5 tiles combined with her 1500-1800 attack will severely cripple anything hit, but it's especially damning to ranged operators who, unless heavily buffed, are liable to get one-shotted due to their low HP.^note Nightingale's wide-area RES buffs help massively with operator survival, particulary those with innate RES. However, Nightingale herself is prone to being one-shotted as her low HP value is not enough to withstand the Arts attacks despite also receiving the benefits of own RES buffs and first talent.
  • She will periodically apply Burning Breath to any one of your operators not already under its effect for 30 seconds, where the affected operator will take true damage over time which rapidly escalates in intensity as the debuff lingers. This attack has a global range and will kill just about any operator if they're not backed up by medics.
  • Once she falls under half health, things get even more hectic, as her defense will be doubled to 1400 and her RES boosted to 90%, making her almost as bulky as Patriot (While her DEF isn't as high as Patriot, very few sources of physical damage can actually continuously deal more than Scratch Damage to a 1400 defense Talulah) On top of that, her cooldowns for Stellar Corona and Burning Breath are significantly reduced, meaning your healing will be swiftly overwhelmed while Talulah spams wide-range Arts on everything nearby.
    • In the Challenge Mode condition of JT-8-2, she starts off in this enraged state. Not only does this mean she can instantly spam Burning Breath like no tomorrow before even engaging combat (and later spam Stellar Corona to instantly wipe your team), but it's even harder to kill her since she'll have her inflated defenses for the entire battle.
  • When she's first battled, there is also an Active Originium tile on her path. And if she reaches it with a reasonable amount of HP in stock, you're done for, because the +50% ATK and +50% attack speed will let her swiftly obliterate any defense you can muster, particularly once she's enraged and spamming nuclear Stellar Coronas.
  • We also haven't discussed the fact that Talulah is fought on the left half of the map. The right half is dedicated to fighting mooks, and there are also a lot of them. You need to leave behind at least 2 and possibly more operators to take care of the Marauders and Shock Troopers coming from the right, leaving you with less operators to fight Talulah with.
• As the True Final Boss of the Reunion arc, The Deathless Black Snake takes everything that made Talulah difficult up a notch and becomes a borderline SNK Boss.
  • Her basic attacks can hit Melee or Ranged operators, deal massive amounts of true damage, and apply Burning Breath on hit, dealing increasing true damage over 30 seconds.
  • Your operators affected by Burning Breath deal half damage to her. Not only is this a problem because she now has many ways to inflict the debuff (including her basic attacks), this means that her HP is effectively doubled unless you can carefully time and position units to avoid her attacks and abilities. Considering that her HP is already among the highest of all the bosses^note 50,000 at minimum, which goes up by 50% when she Turns Red, this makes an already bulky boss even more difficult to take out, letting her shrug off attacks from things that'd normally instantly kill any other enemy, and outright laugh off anything less.
  • Her global Burning Breath attack now targets two operators at once, and she can now cause all friendly units with Burning Breath to explode, dealing massive Arts damage to them and other operators near them. Operators hit by the explosion get afflicted with Burning Breath, though operators that explode lose the Burning Breath effect. She can also use this detonation alarmingly often considering how much havoc it can inflict, and if she has a large number of units affected with Burning Breath when she detonates them, you can say goodbye to about half your lineup since the explosions can overlap. Oh, and on H8-4, if The Deathless Black Snake doesn't have an operator (or summon) to apply Burning Breath to, she will apply it to the civilians which can lead to them swiftly dying and effectively failing the mission.
  • Once she's killed for the first time, Deathless Inferno triggers, summoning 10 waves of fireballs (one fireball per horizontal lane, coming from the left) that hit friendly units for obscene amounts of Arts damage. If you don't have Nightingale then expect your entire deployed squad to be wiped off the map unless you took a lot of decoy units. While this is happening, the enemies will gleefully try to sneak through, which is deadly because any operators you deploy to block them will get roasted by the fireballs.
  • Phase 2 begins here, with The Deathless Black Snake reviving to full and getting a 50% HP and ATK boost, as well as becoming invincible for 10 seconds. She also starts to summon energy crystals that explode into more fireballs (though this time applying Burning Breath and dealing more damage) unless they're taken out first.
  • When you kill her a second time, she gives you a final proverbial middle finger right before she dies for real, triggering Deathless Inferno again and potentially wiping your squad out a second time. Not to mention that at the same time, more Hounds try to sneak past and the second wave of enemies starts to spawn in, which will likely be able to overpower your defense after it's been crippled by Talulah and the Deathless Inferno.
  • Despite the SNK Boss nature of this fight, that is not to say it is completely impossible or require E2 Nightingale. It is fully possible to beat the stage using solely 3 or 4* rarity operators. In fact, someone has done it before, however, as most 3 and 4* operators on their own are not as good as 5 or 6*, players will most likely require advanced knowledge of pattern and timing of the stage down to a tee as well as some skill masteries. In short, prepare for a difficult fight regardless.
• Mandragora from Chapter 9 can be a tremendous pain in the rear mostly due to her Stoneshield, which drastically reduces Physical and Arts damage taken, and can only be broken by dropping a Tattered Pillar on her. The Pillars in question are single-use and not always placed in convenient locations, making it very possible to permanently waste your chances to bring her down - not helping is that the levels she's fought in will often demand some of the pillars be used for rerouting or killing elite enemies, further limiting your chances at breaking the Stoneshield. Once you crack the shield, you have only 25 seconds to lay down the hurt on Mandragora before she grows it back, and she'll also instantly regain it if she takes enough damage at once, just to make bursting her harder. Offensively, she's also a hard hitter with alarmingly fast Arts attacks and Mandragora's Gaze, which deals massive continous damage while crippling the target's ASPD and potentially sealing the tile if the target is retreated under its effect. However, once you take out her first health bar, her second phase is the real kicker, as she'll start to fly while gaining a Flechette Storm aura, ranged attacks, and the ability to manually bring down pillars on top of operators for a One-Hit Kill. The flight can be extremely damning as it'll make her unblockable and immune to non-ranged attacks, rendering her near impossible to stall or stop unless you can bring her down - and bringing her down is now harder since she can fight back at range and use the Pillars against you, wasting your chances of defeating her while disrupting your formation and making it even harder to fight back.^note However, it's worth noting that her new ability to collapse Tattered Pillars can be an advantage, as she can be tricked into collapsing a Pillar onto herself.
• The Damazti Cluster in Episode 12 is a particularly tough boss to handle. To start off, the Damazti Cluster has an enormous amount of HP and always starts disguised (poorly) as a civilian, which means you cannot alpha strike them early with long range or helidrop Operators until you reveal the Damazti Cluster. However, once you do reveal the Damazti Cluster, they have a fast melee attack that deals Corrosion damage to any Operator blocking them, eroding your blocker's DEF despite their only moderate ATK, along with a global-ranged attack to mess with you if you didn't pay attention to your deployment order. When they hit 50% on their first lifebar, they summon a second copy of themselves that shares their HP and abilities, but takes a different path; attacking both Clusters at once is almost necessary to deal significant damage, due to each one reducing individual damage taken for every clone that's alive. When their first lifebar is depleted, they'll generate a large AoE Arts aura that can wipe out your Operators if they are packed too close together (especially if multiple Clusters are stacked), before gaining a Herd-Hitting Attack that nukes all units for heavy Arts and Corrosion AoE damage while spawning drones that will harass your units with even more physical and Corrosion damage. Finally upon hitting 50% on their second lifebar, the Damazti Cluster will target the last deployed Operator (prioritizing the one with no allies in adjacent spaces), and instantly knock them out for the rest of the level, and then summon a third copy of themselves. Obviously, the challenge comes when the Damazti Cluster and its copies begin pushing down separate lanes while periodically nuking a target with their single-target ranged attack or blowing up your team with their special move, which will inevitably spread your forces thin trying to contain them, which is a problem when each one can be a major threat on its own if left unchecked. To make matters worse, the Damazti has significant invulnerability windows whenever summoning a clone or while a clone isn't revealed yet, wasting your skill uptimes and giving the Clusters more time to tear into your units. On top of that, Episode 12's challenge maps give the Damazti Cluster a major stat buff and will upgrade their abilities, such as buffing their Corrosion damage, reducing their skill cooldowns, or most damningly allowing them to summon three copies instead of two, meaning you'd have to deal with four souped-up Damazti Clusters simultaneously. Obviously, Therapist Medics, high DEF Defenders, Fast Redeploy Specialists, and AoE damage dealers are highly recommended.

Side Stories and Events

• The Heart of Surging Flame event introduced Pompeii, who will quickly teach players not to underestimate it just because it's an Originium Slug.
  • From the get-go, it has a massive amount of HP, and its whopping 70% RES makes most Casters all but useless against it. That being said, it has low DEF by comparison, making operators who deal high physical damage ideal to use against it. Both Hellagur and Jaye in particular trivialize the fight due to often being able to outpace Pompeii's damage output with their own self-healing, while Hellagur is dealing heavy burst damage in turn with his skills, and Jaye can cut it down easily with the help of his talent.
  • However, its main claim to power is its ability to simultaneously hit four operators at once with its Arts attacks. Ever wondered how Reunion mooks feel being roasted by Eyjafjalla? Because it will teach you. While it has rather low attack for a boss, its fireballs also inflict a nasty burn debuff that can quickly pile on damage when combined with its constant barrages. All of this allows Pompeii to ravage groups of operators at once from a range longer than most casters, and Pompeii dealing Arts damage means that it's going to be hitting right through your units' DEF. It's basically the same as Eyjafjalla's Volcano, but running on infinite duration. If you don't pile on the Medics, be prepared to suffer a Total Party Kill or two when fighting the behemoth.
  • Not to mention that when Pompeii is blocked, it is able to unleash a devastating AoE eruption that deals an obscene amount of Arts damage, potentially instantly killing all squishier units in the vicinity.
  • Once it hits half health, Pompeii gains a substantial attack speed boost. If not killed quickly after this point, it can handily bury your entire team under its rapid-fire lava volleys.
  • Pompeii also served as a nasty Early-Bird Boss due to its home event being the second event released in the entire game, ensuring that many players lacked the resources or units to overpower it, as well as making it predate the existence of many units that can easily handle Pompeii now, like the aforementioned Jaye. It's telling that when the Obsidian Festival Rerun rolled around, many players thoroughly enjoyed returning with much stronger forces to tear it a new one. Because of this, one Pompeii isn't as difficult as it used to be...so FA-6 from Rewinding Breeze rectifies this problem by sending a second Pompeii.
  • From Operation Pine Soot onwards, Pompeii is also featured in one of the daily maps with five separate risks that can stack with one another if they happen to appear together:
    • Tough Constitution buffs its ATK while massively buffing its DEF and RES, making it take Scratch Damage from nearly everything (in particular making it borderline immune to Arts) and making it impossible to kill in a timely manner, giving it more time to rain hell.
    • Internal Eruption buffs its HP to make it more durable and causes its eruption to both deal massively increased damage with a significantly reduced cooldown, meaning that trying to block it becomes a death sentence for everything in the vicinity given that nearly nothing can hold up to the damage from the constant eruptions.
    • Desperate Survival buffs its HP and heavily buffs the attack speed buff it gets in its berserk state, meaning that the instant it hits half health, it'll quickly douse your entire team in fireballs to decimate them.
    • Unstoppable Force increases its ATK and massively buffs its range while letting it No-Sell all statuses and making it invisible to boot, letting it snipe your units with impunity and forcing you to block it, letting it use its eruption.
    • Viscous Flame heavily increases its Damage Over Time and extends its duration up to infinity, which combined with its normal attacks can easily spike its damage output to unreasonable levels.
• The Rat King from Code of Brawl can be a tough nut to crack, not necessarily because of his stats, but because you absolutely have to devote a good chunk of your unit placement if you don't want to get destroyed by his skills.
  • Every 15 seconds, he'll hit the Operator with the highest max HP on the map with Singing Sands, a veritable nuke of an attack that deals very high physical damage in a large cross. If they're a melee operator, they'll probably be able to take it, but its range is so massive that squishier ranged units will almost certainly be caught in the blast unless you took measures to put them out of the way beforehand.
  • He'll also periodically hit the lowest HP unit with Sand Tomb, which deals Arts damage over time in a wide area. While its damage will kill units without medic support, it isn't too bad, but the problem arises with the 70% ATK debuff it nails any unit within the tornado with. Since it targets the lowest health unit, it'll almost certainly be hitting a damage dealer, severely lowering your damage output against him and his flunkies. Worse still, it affects Medics, meaning your healing will also be neutered.
  • His skills have a global range and can't be stopped, meaning that your formation absolutely has to be built around him. In this process, you risk leaving other paths open for attack, especially when he's accompanied by stronger enemies that will engage you from the other side of the map.
  • As an Arts caster with above-average ATK, he's going to be punching holes in your units whenever he's not casting his abilities, although his durability is on the low side for a boss. That would be a significant weakness if he didn't also spawn with a barrier that gives him 3000 DEF, making it borderline impossible to burst him down with physical damage. You can take out the barrier with Arts, but considering that it takes anywhere from 14,000 to 22,000 Arts damage before going down coupled with Sand Tomb debuffing your casters, it might take a while.
  • Once he's below half health, all his attacks and abilities will start dealing more damage. You best hope you're prepared to finish the job, because a boosted Singing Sands or Sand Tomb will hurt.
• Twilight of Wolumonde allows us to get acquainted with Mudrock pretty quickly, whose already massive bulk is amplified even more by virtue of them having a barrier skill like the Rat King. It gives the boss significant HP and ASPD buffs as long as it exists, and while it can be taken out with Arts damage, unlike the Rat King's shield it will be periodically refreshed. Not only that, stalling the boss is nigh impossible for long periods of time without any invulnerability skill/talent or slow debuffers because Mudrock's Gathering Steam ability permanently increases their ATK every time they hit something, letting them stack attack power like no tomorrow up to a whopping 360% ATK buff. Coming off of a minimum of 800 ATK, that ATK is no joke especially with the ASPD boost from their shield, letting them instantly kill most Operators sent in to block them. However, what cements the boss as a difficult one is the fact that they can instantly hijack any Gramophone tower closest to their current location every 35 seconds after spawning. With how important Gramophones are in taking down Colossi and preventing enemy healing, it is imperative that that the player take down Mudrock as fast as possible before the situation quickly grows out of control. This is most definitely not a trivial issue, due to all of the aforementioned issues listed above, plus Mudrock can potentially receive significant amounts of healing from any Gramophone tower they hijacked to undo all your hard-fought progress.
  • Mudrock returns with a vengeance in Operation Lead Seal's permanent map as part of a Dual Boss with Big Bob. While their HP is slightly lower and they don't have any gramophones to hijack, they're still a massive threat since their stats will be buffed by the risks you're taking on top of their pathing taking them over an Active Originium tile^note Buffs their ATK and ASPD by 50% at the cost of them losing 150 HP per second, which is a pittance since Mudrock in that stage has 25,000 HP at minimum- particularly, Heavy Armor, the contract that buffs them and Big Bob will significantly increase all of their stats while making their Deflector Shields both significantly stronger and faster-regenerating, making them even deadlier while giving them an unholy amount of bulk. If Mudrock manages to meet up with one of their Colossi, Big Bob, or both, you're almost certainly dead unless you have some nuclear area attacks that can deal with their insane combined bulk.
    • Oh, and about there being no Gramophones for anyone to hack. There are Mudrock Colossi on the map, and you're going to have to figure out how to nuke them down. While this means Mudrock won't be turning them on you, it does mean you need crazy DPS to take down the Colossi and likely won't be able to structure your team from taking down Mudrock, who's still as absurdly overpowered as ever.
• The Essence of Evolution from Operation Originium Dust was considered among the most annoying bosses in the entire game shortly after its release, and for good reason.
  • Despite it being a Stationary Boss, it makes up for this by being both a multi-stage Sequential Boss and a Flunky Boss of the highest degree, passively spawning in large amounts of Originiutant Excrescences to overwhelm defenders. While they're weak individually, the boss will eventually replace them with Originiutant Tumors which will reduce blocking Operators' attack speed by 80%, making it nearly impossible for them to fight back properly and thus making it easier for the hordes to slip through; and we do mean hordes, because the boss will start spawning them at a terrifying rate later in the fight, on top of the map also featuring huge numbers of Mook Makers that turn the spam up to eleven.
  • While the boss won't attack normally, it will periodically use a devastating attack where it strikes the entire map with a shock wave, dealing heavy true damage to all deployed units. DEF and RES is both useless against this, meaning that you'll have to have healing support or self-sustain on all of your units (which in itself is difficult due to the map layout; see OD-8 under That One Level) if you don't want them getting blasted into oblivion. Worse still, as the boss evolves, its shockwave will both get more frequent and more damaging, becoming nonstop in its last phase - this makes setting up the Originium Explosives (which deal massive damage to it and clear out its minions) difficult since the boss will often just kill them before you can arm or detonate them.
  • One might be tempted to burst it down like a usual boss, but the Essence of Evolution circumvents that with a Puzzle Boss shield rendering it incredibly resilient to damage from the left side of its body, which swaps to the right side of its body after it evolves once. This means that bursting it down needs you to have two sources of burst damage to deploy on either side of it as it evolves, all while you're handling the oncoming Zerg Rush - and even that's not enough, because when it's on it's last legs, it'll become borderline immune to conventional damage^note True damage like Amiya's Chimera or the Originium Explosives given on the map bypasses all of its shields and force you to experience its painful final phase no matter what (although its health burns away over time to compensate). And before you think about stalling it, it'll evolve by itself if left alone for a while, which only gives it more health to burn in its second and last phases.
• Pursuer from A Walk in the Dust demonstrates exactly why the Ursus Royal Guard is so feared. While his stats are largely average, Pursuer's main threat comes from his special ability to create Dominion zones on the map every time he loses a portion of his HP, and always centered on the operator not within Dominion that has dealt the most damage to him. When operators are within Dominion zones, they suffer a significant ASPD debuff, while Pursuer takes significantly reduced damage from physical and Arts attacks when inside Dominion. In addition, any operator within Dominion that is hit by his Collapsing Fear AoE attack is instantly retreated. There is no exception to this; even abilities that render operators invincible like Mudrock's shield or Specter's Bone Fracture cannot protect them. This means that operators stuck in Dominion are severely hamstrung and vulnerable, making it borderline impossible to maintain a defense without retreating and redeploying to work around Dominion, and trying to attack Pursuer while he's inside Dominion is basically a waste of time unless you can deal true damage to him. Not helping is Pursuer' second phase which opens with a global Collapsing Fear blast (instantly killing all Operators in Dominion when he switches phases) and lets him deploy Dominion every quarter of his HP lost instead of half, meaning you have to deal with him casting Dominion at least 4 times over the course of the fight; not to mention that when he Turns Red, he gains an ATK boost and starts using sword attacks that deal upwards of 2000 damage, making it damn near impossible to block him without a dedicated tank. Also, Dominion is a permanent effect, so you can't simply stall out the fight to wait for it to disappear; this can get very problematic when Pursuer is defeated and your forces are left crippled against any remaining enemies, particularly the Elite Imperial Strikers that show up after his death in WD-EX-8. Did we also mention that Dominion is massive? Every time it is applied, expect 1/8th of the entire map to be covered in it.
• The Blood Knight Dikaiopolis from Near Light gladly shows off how he earned his status as the Major's champion. He has extremely high ATK that can instantly crush any operators without sufficient HP and DEF, and has 25% damage reduction against all operators who are not blocking him, excaberating his high HP and defensive stats. He also perodically uses Exsanguination, a global-range Arts Damage Over Time that also Stuns the target, letting him waltz right past blockers if there's only one stopping him. However, the real fun begins when his HP first hits 0 and he enters his Resurrection Stance, where the focus shifts to stopping an onslaught of summoned Bloodblades from running into him and healing him on contact. Failing to stop at most five Bloodblades will cause him to fully heal, re-enter his first phase and force you to do it all over again, a problem compounded by the Bloodblades having fairly high stats and being able to carve up squishier Operators without healing support. Even when you do stop the Bloodblades, his second phase significantly increases his already towering ATK to let him demolish nearly anything blocking him, lets him attack ranged units (which will almost certainly kill them instantly considering his ATK is well over two thousand at this point), and doubles his damage reduction against targets not blocking him to make him even tankier, along with constantly spawning Bloodblades all over the map to harass you and potentially heal him for large amounts of health if you don't intercept them in time.
• Similar to Dikaiopolis, Degenbrecher the Black Knight from Break The Ice proves her worth as a tough opponent deserving of her Kazimierz championhood and legendary reputation. While she's fairly fragile despite hitting very hard as per most bosses, she has a multitude of painful abilities that make bringing her down easier said than done. Firstly, her Swift Slayer is a dash similar to Crownslayer, except Degenbrecher is immune to silence and too heavy to be reasonably shifted, meaning she essentially cannot be stalled and making it a race to take down her before she reaches the Protection Objective - the dash also deals heavy damage to everything it passes through, which is compounded by Degenbrecher's passives that let her ignore 20% of the DEF of units blocking her, and gain a substantial ATK boost if Snowfall is currently active. Ranged units aren't safe either, as her Brutalizing Blizzard will periodically slice up everything around her, with her good ATK stat meaning that squishier ranged units will really feel it. But the true fight begins once you knock her down and bring her to her second phase, where she becomes invisible, starts hitting twice with all her attacks, strengthens her DEF ignore and Snowfall passives, and gains a nasty ability in Momentum Murder which disables all nearby operators and drains their SP each time she loses a quarter of her health. The invisibility means that without a source of decloaking, ranged units won't be able to gun her down like in the first phase, forcing you to bait out the dash and then block her before bursting her down. However, Momentum Murder also means that burst and healing options will be stymied by the wide-ranged disable as few options are long-ranged enough to constantly hit her from outside its radius, leaving you vulnerable as Degenbrecher either tears into your units or stalls long enough to get her dash back, skipping your defense entirely.
  • Ironically, her boss, SilverAsh, is effective against her due to his Elite 2 talent revealing invisible enemies in his attack range while having a range barely wide enough to hit Degenbrecher without being affected by Momentum Murder. Totter is also effective due to his anti-stealth gimmick and Besiegers having long enough range to snipe Degenbrecher outside of both her attack range and Momentum Murder.
• Andoain the Martyr from Guiding Ahead is similar to Pursuer in that what he lacks in raw stats, he makes up for with heavy area control. As long as he has ammo, his basic ranged attack deals heavy damage and can bounce up to five times, heavily discouraging you from grouping your operators too close together and severely punishing you if you can't manage the Gelato Stops to stop him from reloading. However, his biggest threat is Light Unto Sufferers, which permanently places down a large light zone whenever he loses a portion of his HP. This zone buffs himself and his allies with physical and Arts damage resistance as well as 50% physical and Arts dodge as long as they stay within the zone, making dealing damage to them in the zone a severe headache. Furthermore, when he's in his second phase, not only will his ATK increase further and his HP threshold for Light Unto Sufferers decrease, he will also start occasionally casting a dangerous Herd-Hitting Attack which allows him to hit the five leftmost targets on the map for massive physical damage, while also reloading any lost bullets. All of this means that Andoain can wipe out fragile backline operators, but his biggest threat remains the massive defensive buffs conferred by Light Unto Sufferers; if he places too many Light zones down in inconvenient areas, your operators may be hard pressed to be able to kill him or his waves of allies before they overwhelm your defenses.
• The Final Bosses of Fire Within the Sand that pop up in Day 12 and march to your base are absolutely brutal, with inflated stats and potentially deadly mechanics that make them tough to fight, although their HP is thankfully carried over in between battles. However, they also come with a veritable army of over 150 enemy units that include a good number of Demonic Spiders in their ranks. Unlike most encounters in Arknights, these units don't come in trickle allowing you to kill them piecemeal, but will come in full force from the start to overwhelm your defense line. And unlike Integrated Strategies or SSS, the buffs obtainable in RA are a lot more modest in comparison, so you're in for a tough fight no matter what. It says something that you are not expected to defeat this army in one go, but rather to raid and whittle their number down as they make their way to your HQ for the final confrontation, and hope you've weakened them enough to finish them off. To elaborate on each specific one:
  • Dauntless Linebreaker is the most straightforward of the bunch, but he's also a terrifying Lightning Bruiser with a total of 1.4 million HP split over his two phases on top of halving incoming damage when unblocked, making him even tankier and reducing your ability to whittle him down before he makes it to the defensive line. Blocking him to disable this is already slightly painful due to his fast and powerful melee blows, but after he revives, his ATK and ASPD will be doubled along with him becoming almost three times as fast, meaning he will almost instantly destroy anyone short of a dedicated physical tank with a lot of healing to back them up. To make matters worse, he comes with absolutely ferocious backup in the form of dozens of Demolitionist and Shieldguard Leaders to soak and deal even more damage, and just as many Tiacauh Rippers, Agent Leaders, and Rampaging Rotchasers whose debuffs will make the hordes of physical powerhouses and the Linebreaker himself even harder to manage.
  • The Ruinbringer is far weaker in a direct fight, but is instead a Mook Maker who will constantly spawn Infused Originium Slugs to assault your lines, dealing massive splash damage to both your units and any vulnerable fortifications that you might have put up to protect the base. His low attack power makes it pretty easy to isolate him away from his army, but said army will easily pick up the slack - hordes of Sarkaz Greatswordsman and Possessed Heavy Defender Leaders act as a beefy frontline for the dozens of Dublinn Evocator Leaders summoning deluges of fireballs to reduce entire swathes of land into craters, as well as the Lord Ameer's Shamans that can stack their Arts Damage Over Time in the chaos to melt your frontline along with the fireball and exploding spider spam.
  • Al-Rafiq is a Mighty Glacier with extreme durability, powerful attacks (including a Charged Attack dealing splash damage), and the ability to self-destruct on death, dealing massive damage and a stun in a wide area to possibly let his allies through. After this, you have 30 seconds to deal roughly 100k damage to the pilot that just fell out of the machine, which is easier said than done when he's invisible and also has an Attack Reflector that can instantly take out fast-attacking units. Failing to do so will result in the pilot repairing the mecha and reviving with full HP, turning the fight into a painful DPS check. On top of that, his army includes extremely tanky powerhouses such as elite Mudrock Zealots, Originiutants, and Maddened Vein Guardians, the latter of which are especially painful due to their own Attack Reflector taking out units before they can even try to fight Al-Rafiq himself.
• From Come Catastrophes or Wakes of Vultures, "Clip" Cliff gladly demonstrates that he can back up his legendary reputation as the leader of Blacksteel even in his old age. His stats are extremely high all across the board, most notably a titanic 2500 DEF and a decent 60-70 RES to make him highly resistant to both damage types, and outright immune to all but the very strongest sources of physical damage. His offenses are equally towering - he has a very damaging ranged attack that inflicts a short-lived but stacking DEF debuff on hit to let him punch holes in defenders, as well as Quickdraw, which inflicts an AoE stun before nuking the main target with three consecutive shots, instantly killing anyone short of a dedicated tank. The player is intended to lure him into Waste Heat Spouts to weaken him with a crippling 70% ATK and DEF penalty before going to town, but the difficulty of doing so depends on the current situation of the level and what units you have deployed to maintain the Heat Spouts, and it's difficult to precisely time the vents due to their somewhat unreliable skill cycling, especially since Cliff will forcefully reset all of their charges when he enters his second phase. Speaking of second phase, Cliff's is a doozy: he gains a decent 20% ATK buff, another 1000 DEF to all but force you to use the Heat Spouts to deal physical damage to him, and doubles his attack speed, which combined with his already punishing attacks means that units that could have tanked him before will comparatively melt doing so now. However, the real kicker is that Quickdraw is replaced with Stroke of Thunder, a six-shot variant that also has the added effect of blinking Cliff forward and resetting its cooldown if its target is knocked out or retreated, which they almost certainly will be due to the sheer amount of damage it does. Let Cliff pull off one too many Strokes of Thunder, and he can repeatedly recast it to nuke your team one by one, while dashing right past anyone that lives. Neutralizing Stroke of Thunder with the Heat Spouts is almost essential to survive it and stop him from going on a rampage, which only makes managing the Heat Spouts even more daunting as synchronizing them with Stroke of Thunder can be the difference between Cliff being walled by a tank or causing a swift Total Party Kill.
• Design of Strife features the final Diablo Sarkaz: Sverdsmeltr, Guardian Ember of the King. In his first form, he deal extremely strong Arts melee attack which will down any defenders in one or two hits, he also periodically use Suppression to incapacitate one Operator, and if your Operator retreats or defeated, they will be sealed until the Haunting Flames are destroyed. Despite that, you can take advantage of his Suppression to relocate your Operators without the need for redeployment time. Once his HP reach zero, that's when the troublesome part begin, he will use Payback to engulf everyone around him with Haunting Flames and releases Condensed Heat towards your Operator until he switches to his second form, where he will summon Reforged Retinues and continue releasing Condensed Heat. When his HP reaches 50% or all his Reforged Retinues are defeated, he will absorb all the remaining Reforged Retinues and recovers his HP and it became even worse, he now fires hard hitting AOE Arts attack while using Extinction on an Operator to incapacitate, which now spawns a stronger Condensed Heat that needed to be taken down before it instant-wipe your whole defenses and it will use Sunwielder's Sanctuary after Extinction to protect against Arts damage while given a massive boost in DEF and dealing Arts and Burn damage around himself.

Integrated Strategies

• The Lost Colossus can be a real hair-puller and has garnered a reputation as a run-ender. While its DEF is decent at best and its RES is abysmal, its titanic HP and very strong ATK still makes it very difficult to take down, especially since its status as a relatively early boss means you may not have very strong Operators yet. However, what cements it as a Goddamned Boss is that it can periodically throw a ranged attack that deals massive damage and stuns the target for a whopping 25 seconds. While it won't target anyone who's already stunned, it doesn't make it much less threatening - the attack has a cooldown of only 13 seconds, so one person will be stunned at any given time. Furthermore, this quirk can be quite a liability as you cannot keep it busy by just throwing a body at it that it can stunlock. Using this ability, the Colossus can easily cripple key DPS units and prevent them from fighting back, or stun your blocking units so the other enemies can walk right through them. It'll also typically start at a location where many Casters can't reach him without putting them in grave danger themselves, prohibiting you from taking advantage of its low RES.
• The Rusthammer Warrior from Ceobe's Fungimist, despite being a Palette Swap of Faust, has certain differences that arguably make him worse. While he can no longer deploy turrets, he has higher stats and a critical hit that deals Arts damage, letting him One-Hit Kill tanky units that would normally laugh at Faust. Furthermore, his backup is also substantially stronger than the minions that usually accompany Faust into battle, which includes a Frost drone. It is fairly impossible to have someone who can tank all the damage plus the Arts bolt, and if you encounter him, you can only hope you have a Fast-Redeploy like Gravel or a Summon Supporter like Scene in your roster to take the Arts bolts.
• Both of the secret bosses of Ceobe's Fungimist can be hair-pulling, especially because the mode's nature as a Roguelike means you might not have all the tools to combat them. Fittingly, they're both heavily upgraded Moveset Clones of other bosses on this list. And if you thought you can just let them through because Integrated Strategies doesn't deduct rewards for leaks, they inflict a life loss of 30 to make sure that thought remains nothing but a thought.
  • The Frozen Monstrosity is a caster boss like FrostNova who inflicts the cold status on all her attacks and can also use a highly damaging AoE burst to deal heavy damage all around her which is doubled against frozen targets, except hers hits even harder than FrostNova's. Unlike FrostNova, however, she has 1500 DEF and 80 RES at base, letting her take Scratch Damage from just about everything and making her low HP a non-issue. Like FrostNova, she still retains annoying abilities like summoning Originium Ice Crystals to help her freeze Operators, coming with a slew of Yeti Squad enemies to exploit the ice elements on the stage, and reviving herself with a period of invincibility upon death - but not only does her revive now give her a massive ATK and HP boost, making her tankier and harder-hitting, but she's also invincible for 80 seconds when first spawned to ensure you can't spawn-kill her. But the worst part is that if a unit under the frozen status is retreated, she doubles their redeployment time - considering that every Operator in the squad is usually important for this mode, and that most Operators on the level will die from being frozen, this can quickly sap your reinforcements until the boss wipes out your squad and you find out that you have nobody left to stop her.
  • Meanwhile, Gravestone is a variant of Skullshatterer with much higher stats to make him even deadlier. He retains the AoE ranged grenades that deal much heavier damage than Skullshatterer's did (although they lose the DEF-shredding property), as well as a melee attack that can take down most melee operators in a few hits at most. In addition, he adopts Mudrock's regenerating Deflector Shields, which now withstand damage equal to 10% of his max HP and protect him from all damage types instead of just Arts (although he doesn't get any buffs while it's up). But the kicker is that his stage has a challenge condition that halves all operator ATK and DEF, cuts DP regen in half, and doubles all redeployment times. It's like having to fight an already powerful boss with an array of Contingency Contracts active - suddenly, all your units are severely crippled, and Gravestone goes from hard to nigh-unstoppable unless you got yourself a team built to counter him, and even then expect him to tear through a good chunk of your team before going down. Fortunately, his backup is usually not a problem as long as you kill off Gravestone before the last few stronger waves start advancing.
• Introduced in Crimson Solitaire, the Wandering Puppet can bring an abrupt end to your run if you lack healing, as its special ability inflicts continuous Arts Damage Over Time to the whole map unless it's blocked. This is usually an instant run-ender if you didn't manage to pull a Medic, and even with Medics you might get overwhelmed due to how high the constant damage is. On the other hand, blocking it to stop the damage is another story as not only does it spend most of its time on an isolated side lane of the map that has very awkward deployment positions, it boasts fairly high stats all around that make it difficult to kill without high burst damage, particularly a very fast and damaging melee attack that can roll over most units without powerful healing support. Although its map gives you a healing tile, it's not enough to outheal the Puppet by itself, and is basically needed to fend off the hordes of Arts Guards and Casters that arrive soon after the boss.
  • This boss practically necessates a good duelist operator who can 1vs1 it, and a good Medic to keep them in shape if they can be healed. However, with the boss lingering a while at its tile before finally getting going, you'll need to bite the bullet trying to hold down the fort in the bottom lane before you can even block it. The usual strategy is to take your strongest melee operator and engage it in a duel, but that requires babysitting that unit and preventing them from dying, since the puppet's damage will far outclipse just about anything you can throw at it. Only a unit with incredibly strong regeneration can survive, and even then the puppet might just outdamage them anyways. Jaye can solo this boss, but he needs to be promoted and his skill turned on before engaging with it - otherwise, the boss will easily cut him down and go on like nothing happened.
• Players trying to get their first clear of Crimson Solitaire will meet a harsh gatekeeper in the form of the Final Boss, Lucian the Blood Diamond. His HP is through the roof, and he passively gains 40% Physical/Arts evasion when not blocked, making it difficult to deal damage to him. On the other hand, blocking him to disable the dodge is a whole other can of worms, as his attacks deal tremendous damage, hit relatively fast, and ignore 40% DEF alongside inflicting Nervous Impairment, meaning that anything he focuses on will usually either die or get stunned; if you can weather his attacks, he can still only be blocked for a maximum of 15 seconds before he blinks through the blocker and spawns a clone of himself to duplicate his attacks, making it nearly impossible to stop him. High ground units aren't safe either, as Lucian and his clones will frequently use a sonic wave that deals heavy physical damage and Nervous Impairment in an area, which will stack up to decimate a whole chunk of units in an instant, especially your squishier ranged units. Lucian himself is a handful, but his backup makes things even harder - particularly the swarms of Scarlet Singers that will rain hell on you while buffing the ASPD of everything on the map, and Heavy Defender Lieutenants that have ridiculously high stats and can overwhelm most defenses by themselves. The final insult is that the path Lucian takes is obstructed by two Roadblocks that he'll destroy when he gets there - unless you defeat him before then (which is much easier said than done), the destroyed Roadblocks will change the enemy pathing to make the Lieutenants and Singers beeline straight to your base, making it difficult to stop them in time and possibly ending your run before Lucian even gets through.
• Continuing the tradition of Ceobe Fungimist, the secret bosses of Crimson Solitaire not only require RNG to encounter them, but the fights themselves are anything but easy; on top of already handicapping the players a fair amount with the requirements to encounter them, each one is fiendishly difficult and requires a specific strategy to beat, which the player may not get due to RNG.
  • The Big Sad Lock is a Stationary Boss that is so enormously powerful that it requires players to have certain specific operators, collectibles, or both just to stand a chance. A massive spike in difficulty from the boss that precedes it, it greets players with a colossal 300,000 HP that regenerates over time (the latter of which can thankfully can be disabled with a collectible, albeit one that requires some RNG and life loss) paired with respectable DEF and RES, making it arguably the most durable target in the entire game right from the start. It's offensive prowess isn't left behind, capable of hitting two Operators at the same time anywhere on the map with powerful physical blows; to make matters worse, every 20 instances of damage it takes, it will pulse and deal global Arts damage to every Operator on the map. This counter includes things like poison ticks from Blue Poison and Thorns, and deals more than enough damage to to kill weaker ranged ops that haven't been buffed with HP collectibles, especially if your strategy revolves around high attack speed that will just make the Lock spam its shockwave. Despite being the only enemy on the stage, it won't stop spawning enemies as long as it stays alive, meaning you can't just focus on the boss. However, the big bear decided that wasn't enough, and also boasts a periodic 15% HP shield, which speeds up all enemies on the map and generates troublesome mobs at an exponential rate as long as the shield exists; combined with its titanic durability, it's capable of completely neutering or even reversing your progress, stalling you until the enemies overwhelm you or 8 minutes pass and the Lock self-destructs to hit you for -30 LP, usually ending the run instantly. Notorious for being considered more difficult than Lucian or even the Troupe Mouthpiece, players will need to hope that their run RNG includes two AoE healers at E2 or ASPD relics, and a maybe a few burst damage Operators with low cooldown otherwise they might as well forfeit when they reach the boss stage.
  • The Troupe Mouthpiece is the most unique boss in Arknights, but that just means the player is more likely to get blindsided. Just to encounter the boss, like Gravestone from Fungimist, the player will have to handicap themselves with one collectible (increases Operator DP cost by 2), which can't be mitigated until late in the run; of course, a secret level on the sixth floor is more than happy to end their run too. Then the boss level itself rears its head; from the get-go, it has a condition that halves natural DP generation, and boasts an entourage of Demonic Spiders like Derisive Audience, Scarlet Singers, and Theater Golems, the former of which can do massive Nervous Impairment damage. Meanwhile, the Troupe Mouthpiece himself is invincible at all times and can only be damaged by by eliminating the Host Assistants it periodically spawns to harass the player. The assistants initially appear to be flimsy and harmless, but just being in their proximity reduces Operators' attack power by 30% while dealing continuous True damage, and they have reduced aggro to direct Operator attention away from them, making them more durable than they look. With the way the map is designed, it's quite easy for operators to always be in their range, and any Operators that are carelessly placed near the middle will be swiftly undone by the Mouthpiece himself, who uses very powerful multi-target attacks that inflict extra Nerve damage. If the player takes too long eliminating the Assistants, the boss may decide to respawn them at full health as well, undoing some progress. Even after 10 Assistants have been killed to take down the Mouthpiece, he'll get back up and enter a second phase where he spawns all of the Assistants simultaneously, potentially overwhelming defenses already occupied by the enemies on the map while forcing players to defeat them all over again; if the Assistants aren't all dead by the 50th enemy or so, the player better haul ass before the Mouthpiece waltzes right into the blue box for a -30 LP deduction. Let's just say the players will need at minimum one Elemental Healer, a powerful Vanguard and a lot of other Operators to throw at the Host's Assistants, all of which requires a copious amount of luck outside of just trying to encounter the stage itself.
  • The Playwright, added in the second major expansion, is just as hard if not harder than the first two secret bosses. His inability to be blocked is complemented by his insanely high durability, and his Arts attacks are simultaneously global-ranged, hard-hitting, and able to deal splash damage to adjacent units. He'll also periodically target all units with explosive Arts that can potentially wipe out entire defensive clusters from the overlapping explosions. However, his most dangerous move is the ability to periodically curse a target, permanently halving their attack speed and dealing Arts Damage Over Time scaling with their maximum HP; if the target retreats while cursed, their redeployment time will be heavily increased and the Playwright will permanently seal the tile they're on a la FrostNova, gradually reducing the space you have to deploy on. And just to add insult to injury, once he gets below half health, he'll enrage and reduce his skill cooldowns while significantly increasing not just his damage output, but his already high defenses (giving him 2000 DEF and 90 RES without reductions), making it difficult to finish him off when he's taking Scratch Damage from most attacks and flattening anything that draws his aggro. All of this is bad enough, but not only does the Playwright spend much of his time loitering in isolated parts of the map while raining hell, he serves as suppressing fire for the Demonic Spiders on his level like the Winterwisp Blood Magisters, elite Originiutants, and Theater Golems, with his interference usually being enough to let them roll right over your crippled defense. On top of all that, even getting to him requires you get two painfully rare encounters that force you into difficult battles, potentially ending the run early if you're lacking in strength or just pure luck.
• Saint Carmen, one of three possible mid-bosses of Mizuki & Caerula Arbor. Although he lacks the Gemini Destruction Law and Mook Maker gimmicks of the Tidelinked duo and Pathshaper, he makes up for it with substantially higher HP and ATK, along with a Hand Cannon that can one-shot the majority of operators. His gun only has three bullets and he can be disabled during his reload animation to interrupt it, but each shot does so much damage that even on moderate Surging Waves levels, getting hit is a death sentence to even the tankiest unit if they don't have a shield - and Carmen scales very hard with Surging Waves due to his high base stats. The stage gives you bushes to hide from Carmen's wrath, but you'll still have to try and confront him eventually to stop his advance, and he can prematurely wipe out your ranged units if you don't deal with his Tactical Scouts dispelling invisibility. Lastly, his backup consists of hordes of Sarkaz Greatswordsmen and Crossbowmen Leaders, which not only have considerably higher stats than the backup on the other boss levels, but also scale extremely hard with Surging Waves levels because of said stats, letting them overrun you before Carmen even gets to the defensive line.
  • If your Light is low enough, you may fight Saint Iberia instead of the normal Carmen. Even though he only has one shot before reloading, his HP and ATK are nearly doubled, and he'll beeline straight to the blue box instead of looping around. Good luck even slowing him down.
• The secret bosses of Mizuki & Caerula Arbor are more manageable than those of Phantom & Crimson Solitaire... on the lower Surging Tide levels. On the higher levels, however, their exponentially increased stats and the RNG element of Light mechanics potentially crippling your Operators makes their mechanics much more threatening, to the point where they become essentially impossible to brute force and demand specific strategies intended to deal with them.
  • The Last Knight is a ridiculous Mighty Glacier with 100,000 HP, 4000 DEF and 70 RES at base and a mechanic designed to shut down brute force strategies relying on using units such as Surtr or Młynar to burst him down. Whenever he takes damage, he applies 2 seconds of Cold to the source - not per hit, per damage instance, meaning that units who deal multiple applications of damage per hit such as Mech-Accord Casters or Młynar using S3 are immediately Frozen, while units who attack too quickly will Freeze themselves as well. The only forms of DPS that can negate this are damage sources that count as separate from an Operator and can attack while the Operator is frozen, such as Goldenglow's detached S3 drones, Pozëmka's Typewriter, or W's mines. On top of that, he also gains a hefty 75% more ATK against Frozen targets, which even at moderate Surging Tides levels guarantees a One-Hit Kill on most Operators due to his substantial 2000 base ATK that is empowered by the difficulty modifiers. A saving grace is that there is a line of rubble in the Last Knight's path that he must break down before he reaches your melee Operators, during which time you must try to defeat him through his Cold debuff. After the arduous task of chipping his health down to 50% while your Operators are being repeatedly Frozen and having their attack speed reduced by Cold, however, things get worse as the Cold duration he applies is doubled to 4 seconds, all but guaranteeing that even slower attackers will get a second hit in while debuffed by Cold and become frozen. And while one may consider using Lee for his second Talent to negate the Freeze and block the Last Knight consistently - this can work to an extent, but because applying the Talent makes his next DP-drain cycle cost 5 DP instead (4 with his Module upgraded), he will eat your DP up fast without a Flagbearer supplying him with a constant stream of DP to keep his Talent going. If defeating him under these circumstances weren't difficult enough, he revives once defeated the first time (albeit only at 25% HP) and mounts his steed Rocinante, becoming unblockable and turning into a Time-Limit Boss that you must finish off before he reaches your base. Fortunately, he does not attack at this stage and moves at half his regular speed, but his first defeat also triggers the second wave of enemies, which includes Leithanian Psalmists, Remnant Principal Violinists and Retching Broodmothers that are assaulting your units with Arts damage (or Corrosion for the latter) while you are desperately trying to burst the Last Knight down. Fortunately, the Last Knight is vulnerable to status effects and debuffs, meaning applying Stuns, Sleep or Freeze are the best ways to stall him and delay his movement for as long as possible. On the highest difficulty levels, this is essentially required, as units such as Erato, Mr. Nothing (whose Unexpected Strike Talent has time to activate while he is Frozen) and Gnosis become the most valuable Operators to have against the boss due to their rapid application of status effects.
    • It's worth noting you may reduce the Last Knight's DEF by 80% if you get The Knight's Corpus from a fairly common encounter. However, because getting the second ending requires you escort him through the first four floors without him being defeated, and this collectible also affects him as an ally, it ranges from incredibly risky to near impossible to take it on a high Surging Tides level, as the Tide-Hunt Knight can be defeated in a few hits by the buffed enemies (particularly the Retching Broodmothers), locking you out of the ending. To get use out of this collectible requires you to put all your effort into protecting the Tide-Hunt Knight, which is a difficult task in itself.
  • Ishar'mla, Heart of Corruption can be spawn killed by strong Operators on lower difficulties. At the high difficulties, however, it becomes paramount for the player to ensure that it never manages to transform from Skadi into Ishar'mla. While it is using Skadi's form, Ishar'mla is a support unit that cannot attack and heals up to three targets at any part of the stage, with her only notable advantage being her very high defensive stats. However, it will passively generate 1 SP per second, and 25 SP per 3 seconds for each of Ishar'mla's Tears that are present, which are hazard tiles she will periodically spawn around the map which do not go away until Skadi transforms or is defeated. To prevent the Tears from generating SP, units must be deployed on them, forcing the player to dedicate units solely to standing on top of them to prevent Skadi from transforming, which is made more problematic by the Tears also dealing bursts of true damage to anyone standing on them, swiftly taking out anyone without enough bulk and/or healing. If she does transform into Ishar'mla, she no longer heals and her stats are reduced significantly, but she will periodically deal heavy true damage to up to three targets simultaneously in a deceptively long range, which on the higher difficulty levels can almost instantly wipe out anyone that's within range to attack Ishar'mla. Defeating Ishar'mla in this form will only transform it back into Skadi (fortunately at the HP she was before transforming), repeating the process - although her transformed form is pretty squishy normally, letting you burst it down before it can cause too much damage, good luck doing so on higher Surging Waves levels with her inflated stats. Once you bring Ishar'mla down to 50% HP, it becomes enraged, increases its ATK by 50% and immediately generates two Tears, at which point the player may not have Operators prepared to deploy on top of them. Should Ishar'mla manage to transform at this point, you are very likely going to be subject to a Total Party Kill due to the amount of wide-range true damage killing your Operators faster than you can deploy or maintain them. What's more, unlike the Paranoia Illusion or the Last Knight, Ishar'mla is immune to status effects in both forms, making it impossible to stall. It is generally agreed that without a copious amount of powerful collectibles, allowing Ishar'mla to transform at a high Surging Tide level is an instant loss. To make things even worse, getting to Ishar'mla means your Light is almost guaranteed to be zero, due to needing the Bishop's Research^note reduces Light by 5 per battle and Caerula Animus ^note -50 Light upon acquiring and inflicting a Rejection to a random Operator to reach the sixth floor, meaning a significant chunk of your squad is likely to be crippled.
  • Izumik, Source of Ecology, takes things even further beyond any of the previous final bosses. His stats are even higher than the likes of the Last Knight or Ishar'mla, but he is instead an invincible Flunky Boss that initially resorts to summoning ridiculous numbers of Offspring which have insane bulk and can transform into other enemies when blocked, all of which are hand-picked to be Demonic Spiders of the highest degree and buffed specifically for the stage. The player will almost certainly be forced to block them not just because of their aforementioned bulk, but the fact that letting enough of them reach Izumik himself will not only cause him to gain a ton of extra HP and ATK for his second phase, but also inflict massive Arts damage to all Operators on the field to further disrupt things; however, Izumik also moves frequently and specifically paths to intercept the swarms of jellies, making stopping them much harder than it looks. Once enough time passes (or worse, if he absorbs enough Offspring to completely fill his HP), things will get dicey as Izumik puts his gargantuan stats (which may be buffed further by his absorbed minions) to use by fighting back - his attacks do tremendous mixed damage that can one shot many units regardless of defenses, and every time enough time passes or he takes enough damage, he'll unleash a global Arts Damage Over Time and stun to let everything else waltz through your lines, if the damage over time while they're down doesn't cause a Total Party Kill to begin with, especially considering that the DOT scales with his ATK and thus can get massive buffs from his consumed minions or Surging Waves levels. Furthermore, like Ishar'mla, the player will have to gimp themselves early on by taking the Caerula Memory that instantly empties their Light and inflicts a Rejection; unlike Ishar'mla, it also requires that you get back that Light, as the item you get in the actual fourth ending encounter depends on the Light level at the time of acquisition. The strongest item can give you a real edge with its tremendous defensive buffs, but if your Light management isn't in top form, you'll have to settle for the considerably weaker normal item, or worse, the last one that debuffs your units instead. And all of this is at base - if you're on a high Surging Tides level, everything listed is exponentially harder, and you'll need that top-grade item, along with a boatload of other powerful collectibles, if you even want to stand a fighting chance against the sheer pressure on his boss stage.
• Out of the Floor 3 bosses in Expeditioner's Joklumarkar, the Demon's Blade is generally the most feared of them all, following in the steps of the infamous Pursuer. He lacks the sheer bulk and force of the Vinecreeps or the massive range and speed of the Snowpriests, but has a bit of both - high HP and powerful physical attacks that can hit at a range longer than most enemy Casters, with his relatively low defenses and slow speed being his weaknesses. However, instead of spreading Dominion whenever he loses a certain amount of HP, he passively spreads it in the tiles around him, meaning that unless you kill him extremely quickly, there's little you can do to stop him from covering half the map in the stuff and even less you can do to control where it goes. Although he doesn't have a "true" instakill like Collapsing Fear, he has his own version where his attacks do 250% damage to targets inside Dominion, which on higher difficulties is effectively the same thing with a new coat of paint considering his high 800 ATK that is boosted further by Braving Nature modifiers, turning his surroundings into an "Instant Death" Radius for even the tankiest of Defenders. He also takes a long path around his stage to spread Dominion over every single tile, which while initially seeming to be an advantage as it gives you more time to take him down, is far less of a boon as it seems as even if you survive the first lap, there are very few units who can actually fight him to finish him off due to Dominion's massive ASPD debuff and his instant kill attacks. Furthermore, he also comes with support from dangerous foes like Imperial Strikers, Ursus Raiders, and Collapsed Constructs.
  • Rarely, the Demon's Blade may be replaced with its Vagabond variant. Although his range is shorter and he moves even slower, his HP is even higher and the radius in which he spreads Dominion is tripled, letting him douse the entire map in an instant. Fortunately, he does take a less direct path that is blocked by rubble to slow him down, giving the player more time to take advantage of his shorter range and shoot him from afar, although this is made complicated by the fact that even if they survive, most Operators will have their ASPD crippled by the Dominion.
• Matching the pattern of the previous 4th ending bosses all falling into this category, the 4th ending expansion for Expeditioner's Joklumarkar potentially wound up as the hardest one to date. To preface things, the player will have to grab a special item from Cannot and using it to enter a reasonably tough combat encounter to grab a key item, followed by a secret floor after the fifth/sixth which not only has the same grueling battles as the sixth floor, but forces you into an encounter where you must pay a huge resource cost in order to progress to the boss. It costs 3 Anti-Interference Index, 50 Originium Ingots, and 20 Hope to fully pay this off, and failing any of the steps will saddle you with crippling debuffs instead: a global -15% HP/ATK, -4 deploy limit, and -12 Max. Life Points respectively. Sure, you can pay off the price using certain items from the other endings, but this requires you to defeat Eikthyrnir and Crazelyseon on top of the 4th ending requirements. It's only afterwards that the boss itself rears its head, and instead of one boss, it's two, featuring an Enemy Civil War between the conquerors Lugalszargus and Kharanduu Khan. Both of them have titanic stats that are made even more difficult by their different specializations: Lugalszargus is nearly immune to Arts thanks to his 90 RES, threatens at range with Arts attacks, and has a Flash Step that instantly bypasses (and likely kills) blockers, while Kharanduu instead has 5000 DEF to laugh off physical damage, and extremely punishing melee attacks that are amplified by his ATK and ASPD ramping up as he attacks. Both of them also have a wide-range Area of Effect nuke that they'll use to blast each other on occasion, dealing indiscriminate damage and Stun to enemies and allies alike; while this can be helpful to cripple or kill a lot of the enemies, the bosses and tankier ones (or all of them on higher Braving Nature) will likely be able to shrug off the blast, while your units will instead suffer a likely One-Hit Kill without some form of Shield or protection. Unless your team is in absolute top form and prepared in advance for the ordeal, most of the fight will be less trying to kill the two bosses, but just trying to survive as they flatten everything in the crossfire between them, not helped by their backup being genuinely threatening and able to overrun your defense if you're already barely scraping by. Furthermore, there's also the pressing issue of having to balance damage between the two and kill them at roughly the same time, as if you only kill one, the other will enrage and become nearly unstoppable, buffing their defenses, speed, and attacks to high heaven while allowing them to fully focus on killing you instead of their rival.^note In addition to doubling their ASPD and movement speed, the enrage buffs their defenses by 2000 DEF and 50 RES; even on Braving Nature +0, this would result in Lugalszargus having 3000 DEF and 140 RES, while Kharanduu would instead have 100 RES and 7000 DEF. Stacking lives to leak them is even less effective than before since not only will your LP be penalized if you didn't cough up the Hope in the prerequisite encounter, they deduct a total of 60 lives if you let both of them escape.