YMMV / Soul Knight

• Breather Boss:
  • Compared to the other two volcano bosses, Anubis is generally considered the easiest one to face, due to lacking the more obnoxious elements of the other two bosses while generally firing less Bullet Hell, which can potentially lead to players just bursting him down before he can kill them. To his credit, though, he has some stiff competition considering Baby Dragon Bros is widely considered to be That One Boss.
  • Iron Pirate King Level is another floor 3 boss that doesn't put up much of a fight, despite belonging to one of the hardest biomes in the game. The king completely lacks any bullet hell attacks, instead using meteor attacks similar to the Goblin Priest, but they pale in comparison to the ridiculous patterns of other floors 3 bosses. Most of the bullets come from pirate enemies that constantly respawn and have extremely wide shotgun blasts, but can be killed easily at this stage of the game. The main reason why the fight isn't a complete joke is because of the king's shield that gives him 75% damage resistance, but that can easily be removed by making him hook his cannonballs.
• Demonic Spiders:
  • Just about every enemy that has a Taking You with Me attack that causes an explosion or poisonous cloud. The former can incur huge damage and a nasty burn effect, and the latter makes the field hard to navigate while slowing/damaging anything that steps in the lingering pool. Green Spider Champions, Varkolyns, and Tentacles are even worse, as they leave clouds of far more lethal purple poison that both deal damage and inflict the poison debuff.
• Game-Breaker:
  • Just about any challenge that allows for infinite energy. Without having to worry about energy consumption, most weapons rendered less useful due to prohibitive energy costs become game-breakingly overpowered.
  • A particular challenge condition reduces firing speed but increases damage output. Normally, this is considered pretty standard, but when combined with any ranged weapon that fires as fast as you can click (Groundwater, charged weapons, etc.), you're getting a massive damage buff for free. The only downside is that rapid-firing a charge weapon generally burns through your energy.
  • The One Punch is an intentional example. While it has a short range, a long-ish charge time, and a massive energy cost, it has a damage stat of 999, which will One-Hit Kill any monster in normal mode while severely damaging bosses in Badass Mode. The catch is that it's a one-time use weapon - however, save it for a tough boss fight, and you'll effectively be able to start the fight with a severely injured or dead boss. Combine it with a charge reduction buff and the Knight's Dual Wield skill, however...
  • The Dead Star Laser Gun, the signature weapon of the Floating Laser UFO and a rare drop from purple vending machines. It's a 2-cost, 5-damage beam weapon that fires piercing laser beams at a very fast rate. While it's already capable of putting out incredible DPS as it is, the lasers are elemental and can freeze, burn, or poison enemies on top of its damage output. Combine all of these traits, and you have a weapon that can lay waste to most rooms and is highly effective at melting bosses.
  • The Shield. While you have to own the premium Paladin class to get it, the weapon just trivializes so many things. Using it will use 2 energy to create a barrier that blocks all bullets for 2 seconds, and using it again refreshes the time and consumes another 2 energy. However, you can switch weapons while the shield is up - with careful switching, you can render yourself permanently Immune to Bullets while still being able to rain hell on the enemies with your other attack(s). The only downside is that it doesn't protect the player from things that aren't bullets, like lasers, explosions, or melee attacks.
• Goddamned Bats
  • Phantoms. They don't have a lot of HP and are relatively non-threatening. However, they can also turn invisible for annoying periods, killing them a lot more troublesome than they should be.
• Goddamned Boss
  • Both the Giant Crystal and Golden Crabs. While they aren't too bad offensively, they can enter a defensive stance that turns all attacks taken into 4-5 HP. Not only can they do this pretty often, but the telegraph for the guard is also the same as one of their other attacks, making it hard to tell if it's safe to shoot or not. Often, this makes the fight a lot longer than it has a right to be, and if you're using a fast-firing weapon, it's possible to fully heal the boss if you aren't paying attention. Giant Golden Crab is even worse since he has more threatening attacks (which inflict burns) and heals even more with his defensive stance.
  • The Cave Ice Bug can consume the ice chunks it leaves around to heal itself and make itself get longer. It summons a lot of ice chunks and spends much of its time burrowed and invulnerable, letting it drag on the fight for an annoyingly long time if you can't burst it fast. On top of that, it can No-Sell freezing effects.
  • Varkolyn Leader/Void, for being able to go invincible whenever he's in poison clouds. Since he spams these everywhere, and his summoned tentacles also make these on death, he's liable to be invincible for most of the fight. If you don't have the poison buff, this is lethal. If you do, it's just a pain in the behind.
• Minimalist Run: The Bad Pistol Run, which involves clearing the entire run with only the Bad Pistol.
  • Until the garbage can, and by extension no weapon runs, became a thing.
• That One Attack
  • Anything that inflicts Burn or Poison, which causes damage over time. Since you normally can't regain HP, getting hit by one will often result in the debuff chewing through your armor and causing HP damage. The worst part is that the damage is unavoidable unless you have a way to make yourself intangible. And if you're on Badass Mode, debuff damage is amplified - get hit once, and say goodbye to most of your health.
  • Explosions and meteor attacks. They do very heavy damage in a wide area on top of sometimes inflicting burn, and they also inflict enough knockback to send you flying into other enemies.
  • Lasers. While they often have a warning laser that precedes them, they strike instantly, pierce all destructible terrain, and unlike bullets cannot be deflected by melee weapons. The Wave-Motion Gun variant of the attack, which many bosses (and the Volcanic Beetles in the volcano stage) use, is even worse, since it has no warning laser and deals (usually) heavy damage for every second it's in contact with you.
  • Attacks that create chains of shockwaves. Due to their significant knockback, it's not uncommon that they can single-handedly juggle you and result in an instant loss.
  • Devil's Snare and Nian have an attack that instantly pulls players towards them. There is no way to avoid the attack, and the only thing you can do about it is to position yourself so that if the attack goes off, you won't be pulled into bullets or other hazards. Nian deserves extra mention for being freakishly strong, sometimes able to pull you out from behind the corner terrain of his arena.
• That One Boss
  • Let's just get this out of the way - any boss that liberally spams fire or poison attacks, especially those that leave lingering hazards, becomes this without the respective buff to counter it. Examples of this include Goblin Priest, Devil's Snare, Easter Bunny (who spams just about every status in the game), Nian, and Varkolyn Leader. This will either leave you taking constant damage over time (and slowed if it's a poison attack), or turn the arena into a hostile minefield. Fighting a fire-spamming boss without the fire buff is especially bad, as not only does burning to incur more damage than poison but if that boss is the Goblin Priest, Prehistoric Colossus, or Easter Bunny, it means you'll be taking full damage from their meteor attacks, which deal 5 AoE damage and huge knockback on top of inflicting Burn.
  • The Christmas Treant. It already fires off countless bullets and shockwaves in its first phase, but its second phase is when it starts firing homing bullets and using a fast charge attack. While many enemies and several bosses charge, the Christmas Treant is invincible while charging, does it extremely quickly, and deals huge damage - not to mention that it's liable to hit you multiple times in one sitting. Combined with its continued shockwave/homing attacks, it can very quickly wipe out players if they get caught in it. Thankfully, the buff that grants collision immunity makes the player immune to its charge.
  • Headless Knight (Headless Horseman & His Horse). It threatens with extremely fast sword attacks, standard, sweeping laser beams, and poison fireballs that litter the screen with toxic fire, which needs both the fire and poison buffs to fully neutralize. However, the problem arises when it turns red, separating the head and body to allow them to attack individually. With both the head and body attacking by themselves, they can set up extremely oppressive attack combinations that are difficult to dodge and can inflict huge damage very quickly unless you keep both of them in your vision at once. The Head deserves a special mention for being able to frequently charge across the screen, becoming invulnerable, and dealing rapid contact damage.
  • Dark Grand Knight. There's not much to say about him except all of his attacks come out ridiculously fast, do high damage, and flood the screen with bullets, including ones that split or bounce. When he Turns Red and starts cranking out the attacks even faster, you'll probably spend all of your time behind the cover in the corners.
  • While he's a Boss Rush/Purity Rush/CNY-only Boss, Nian is a major pain to defeat, especially if you don't have the fire immunity buff. Not only can he fill the screen with fire and explosives, but he also fires splitting shots to flood the room and bounces bullet lines that form a net to restrict your movement. But the worst part is that he can roar to instantly pull you a massive distance towards him - often straight into swarms of bullets.
  • If you don't have the poison immunity buff, Varkolyn Leader goes from reasonable to nightmarish. Not only can he summon large amounts of purple poison pools - which slow, damage, and inflict the poisoned debuff - he also gains invulnerability while standing in any purple pools. It doesn't help that he constantly summons Tentacles, bulky turrets that also leave purple poison on death. Not only is he liable to fill the entire screen with poison to make getting poisoned inevitable, but it also has the side effect of making him invulnerable almost all of the time - and that's on top of his actual attacks, which include a Wave-Motion Gun that deals 6 damage at a minimum. On the flip side, if you have the poison buff, Varkolyn becomes a Goddamned Boss instead.
  • Zulan The Colossus. In a game full of bosses that love their Bullet Hell, Zulan stands out as the most bullet-hellish of them all, firing volumes of projectiles in complicated patterns to fill the screen at all times as well as linear lasers that come out surprisingly fast. A lot of the challenge comes from the five drones that accompany him - while they usually help him launch his attacks, he can sometimes send them out to chase players with shotgun bursts and lasers. And when he Turns Red, he can continue attacking while his drones are out. Have fun! Thankfully, its 'parallel' counterpart from Soul Prequel, Zulan In Ruins, while sharing its attack patterns, has its attack speed toned down (it's 1-5, after all), making it way easier.
  • The Floating Laser UFO. Not only does he have the melee charge attack that made Christmas Treant and Headless Knight so obnoxious, but he also leaves behind screen-flooding turrets every time he uses the attack. While he's doing this, he will continue to threaten with volumes of bullets, bursts of homing missiles, and his summoning of alien squads to harass you with laser barrages. Good thing Chilly Room added 2 test tubes as covers later on. (you still have to dodge the wall-crawling neon electric balls though.
  • The Baby Dragon Bros, the game's resident Dual Boss and often regarded as one of the hardest bosses to get as your 3-5 boss. Both Dragons have extremely chaotic attacks that span all manner of bullets, lasers, and fire attacks - the latter of which can flood the screen with fire and explosions, making a run without the fire buff almost impossible. And since both of them are attacking at the same time, it's often hard to make out openings in the chaos, which could lead to a very fast death if you aren't on top of your game. The Grey Dragon is even worse than the Black one, as it uses explosions rather than fire fields and can fire a nigh-unavoidable laser barrage below half health.
  • Golem - ANCESTOR, the the final boss of the bonus biome (4-5), piloted by the Fallen Hero High Priest. For starters, there is zero cover. Besides, the 'rock rain' may be not hit hard (1/2 DMG per rock), but they come in about 10-15 barrages like a Deathofa Thousand Cuts. Not to mention the fast laser eye attack (it covers the entire arena save for the top left/right areas). And it gets harder once it Turns Red. Good luck!
  • Both Purity Rush bosses (Sir Sangria/Sir Violet) count as this, considering they are secret bosses. First, you must complete Purity Rush in 12/17 Minutes BUFFLESS & WEAPONLESS (note that you can still pick up weapons later, but you can't pre-select them in Lobby) to even get the right to fight them. As for the bosses themselves, Sir Sangria takes almost no damage unless you kill his Minions (Sword/Bow/Pistol/Rifle Knight), which all have 300HP and get both tankier and more aggressive the more times they are revived, forcing you to kill Sangria quickly while dealing with his homing arrowheads and the Wave-Motion Gun from his Sealed Souls' Warhammer. As for Sir Violet, oh boy: from the get-go, he has as much bullet spam as the toughest 3-5 bosses, notably a rotating laser grid. The problem is once he takes off his cape at half HP - his aggressiveness goes way up, he can throw his Oathkeeper like a Boomerang and his Oathbreaker as a Homing Projectile, and spams all manner of melee and laser moves. Good luck!
  • Both Sacred Weapons Bosses (Shadow Knight/Giant Magma Crab) count as this: Firstly, they only have 4 blocks as cover (similar to Devil's Snare area). The problem is unlike Dark Knight/Giant Golden Crab, the big bullets EXPLODE, dealing AOE damage. Not to mention if your energy runs out, you can only melee the boss, which will be a pain in the ass. Even worse, if you run out after he reaches half HP...you're fucked. Even worse if you chose staves & bows, as they do not have melee functions...