That One Boss / Guild Wars 2

• The Fire Elemental in Metrica Province used to be this, setting people on fire, spawning embers like mad and generally being a pest, until the World Boss overhaul in 2014. Now it's lucky if it dies as much as 30 seconds from spawning.
• Rozgar the Forge in the Citadel of Flame main quest is easily the hardest boss, due to the fact that he starts out with a greatsword and two floating greatsword adds. All three of them can use a whirlwind attack that is fast, covers a large area, hits multiple times, and can do up to 1500-4000+ damage PER HIT. Fortunately, he stops using this once he loses 1/4 of his health and becomes a Goddamned Boss with Teleport Spam once his health drops to 1/4.
• Kudu's second golem in Sorrow's Embrace can come off as a nasty surprise if the players doesn't have a lot of weapon skills that doesn't inflict conditions. The reason? It consumes its conditions to heal itself, forcing the party to rely on raw damage to defeat it, and screwing over players whose auto attacks inflict conditions. Oh, and the golem really enjoy shooting fireballs all over the place that will target and burn you for large damage, and will likely down you if you are standing too close to another player. And just to make it even worse, in one of Sorrow's Embrace's explorable paths, you have to fight both the golem and THE OTHER TWO GOLEMS, AT THE SAME TIME.
• Do we even need to talk about Giganticus "Motherfucking" Lupicus? Present in every single explorable path for Arah, extending the length of that dungeon by an extra thirty minutes to an hour due to his ridiculous amount of health (split into three phases) and super-high damage that guarantees the imminent death of Glass Cannon players and forces you to actually use your dodge rolls prudently. It's arguably the highest-skill-requirement boss of the game at the point of writing. And some are badass enough to take on him solo.
• Simin is pretty much the reason hardly anyone will form a group for the Seers' path in Arah's explorable mode. Her schtick is that she can turn invisible and the only way to make her appear again is to bring enough sparks to the statue of Dwayna. While she's invisible, she's completely invulnerable and regenerates her health. She can regenerate to full health if you don't cancel her invisibility fast enough! She can also petrify people and the only way to cure that is to toss tears at the afflicted players. In order to win this fight, everyone has to know what they’re doing, everyone has to be well-coordinated, and most of all, everyone has to be a Glass Cannon. Otherwise, Simin will regenerate more health than the team can take down. Worse, the fight is notoriously buggy and the sparks have some odd pathfinding issues, making it all the more easy for Simin to fully regenerate.
• The very first boss that will make a group of new characters cry tears of bitterness is "The Lovers", a Mesmer and Elementalist pair is Ascalon Catacombs. They must be kept away from one another or they'll regenerate health faster than you can possibly damage them, making crowd control coordination an absolute must, while also dealing with the Elementalist's damage and the Mesmer's debuffs simultaneously. It may not look so hard on paper, but getting a new group to coordinate enough for it is near-impossible without huge repair fees and lots of frustration. As soon as you discover what the nearby rocks are for, however, the pair gets separated quite easily.
• Turmaine, the last boss in the Seraph route of the Caudecus' Manor explorable mode, can be this. He puts down many areas of conditions which can overlay, capable of downing people if they don't dodge or cure themselves in time. Then, he has a plague form which makes him invulnerable AND deals three separate stacking/intensifying conditions at the same time AND has "super-speed". Oh, did anyone mention his plague form just feels painfully long and merely prolongs an already fairly frustrating level in itself?
• Ulgoth the Modniir in Harathi Hinterlands used to be a pain not due to being arduously difficult, but because having a very short spawning/kill window, where the events needed to lure him out stretched out through most of his available spawning time, leaving simply not enough time to down him. That, however, has also since been remedied, and he is now one of the easiest and most rewarding (4 guaranteed champions that drop loot, huge amount of centaurs that drop craft item bags) bosses out there.
• Then there's the Queen's Gauntlet, which seems to have been deliberately made of this. It would be easier to list the number of matches that aren't frustrating. Most of Tier 3 is a hair-ripping experience, and a number of the achievements for it are often the result of a combination of persistence, trial and error, and luck. Special mention goes to Liadri the Concealing Dark, a boss that can take dozens if not hundreds of protracted attempts to beat. Players seem to agree that possessing the miniature that's awarded for beating her is an outstanding sign of your dedication and skill. For the truly masochistic, there's also the gambit system...
  • Liadri's fight deserves extra mention, since it was a knife's edge between a win or a loss. The fight involved two phases, the first of which had Liadri, who was invincible, tossing out projectiles from the center while dark clones of her walked towards your character. You had to kite those clones towards pools of light, forming pickups which you tossed at Liadri. After 3 hits with the pickups, the second phase starts and she starts chasing you with her obscenely powerful whip while the number of clones spawn faster. By the way, as soon as one of those clones makes physical contact with you, it's an instant kill. There's also a periodic AoE that covers a quarter of the circular stage that also causes instant death, and the AoE doubles to form an hourglass shape in the second phase. Shining orbs will occasionally spawn and pull your character - sometimes saving you from the instant-kill attacks, but more often dragging you into one of them. The fight progresses as a frantic mix of kiting Liadri and her clones, timing dodges for the instant kill attacks, and trying to deal enough damage to win the fight before the timer expires. Many failures were the result of a clone spawning a bit too close to your character, or running out of dodges to avoid the next AoE.
  • The return of the Queen's Gauntlet in 2018 has added new tough bosses to the fray. Of particular note is Turai Ossa. The fight involves three phases, the first two of which is spent almost purely on survival as Ossa has two buffs, Shield of Elona and Eternal Blade that increases the damage he does immensely and makes him nearly invincible. The strategy is to survive his relentless onslaught for roughly 20 seconds, either through evasion, kiting, and/or tanking his damage until he jumps to the center. You then have to dodge his shield throw or he gets extra buffs and stuns you for a long period of time. After dodging, you need to use your crowd control abilities quickly and break his Defiance bar in order to get rid of his Shield of Elona buff. Next, you have to repeat the process once more from the beginning, surviving and breaking his bar once more, to remove Eternal Blade. Failure to break this bar results in him getting back his Shield of Elona and Eternal Blade buffs. After all of this, you can "safely" DPS him, but he still does a heavy amount of damage so it has to be quick, especially on a squishy character. This Troper fell to his punches and kicks at least once because he assumed he would be helpless without his buffs.
• The event Tequatl Rising revamped Tequatl the Sunless, the dragon champion that spawns in Sparkfly Fen. What had used to be a simple "press autoattack and go for a coffeebreak" world event was changed into a tough fight where 100-120 players have to closely cooperate to deal damage (main zerg), support the zerg from turrets (by removing hardened scales buff from Tequatl and buffing/cleansing the zerg from poison) and protect the turrets from spawning mobs. Doable with organized groups, even those arranged near the last few minutes before his spawn, as long as there are enough players (due to the LFG system and low-population maps being filled up). Some time in 2015 though there was a patch that allowed Tequatl to receive critical hits but made his hitbox smaller, making the fight frustrating and doubly increased the chance of failing. Was thankfully reverted back again a few weeks later.
• The "Triple Trouble" giant jungle wurms in Bloodtide Coast are a true nightmare for any group of players that doesn't prepare meticulously for the event, to the point where it's only possible for extremely coordinated, professionally led guilds to down it. It pretty much mandatorily requires three separate groups of 40 players to work on the perfect command of three commanders that have to be in direct contact over Team Speak with one another. It's currently the only event that's pretty much exclusively ran by the TTS guilds, because they alone have the level of organization and manpower to down the wurm.
• Anyone that went for the Belcher's Bluff mini-game achievements know of the extreme annoyance that is Adnul Irongut. For people that don't know, the Belcher's Bluff achievments involves the player facing off against six NPCs, each with signature moves. The match plays out like a drinking game - normal drinks reduces the player's standardized health, but they can also fake a swig, drink water to restore some health (but has a 3-turn cooldown), or belch, causing their opponent to take extra damage if they faked and negating the health restoration of water. Adnul's special move was Critique, which prevented fake swigging for three turns. If your belch or water was on cooldown, this was a death sentence, as you would usually lose all your health after five straight drinks. He also had the nasty habit of using Critque again right after the previous one had worn off. Coupled with an extremely large health pool, the match soon turned into a Luck-Based Mission as the only chance that player had was to predict/guess the turns Adnul would use water and counter with belch, and hope he didn't start spamming Critique. There were some glitches that other players have exploited for easier wins, such as using food/rejuvenation boosters for health regeneration, but they have all been patched.
• Heart of Thorns has the Vinetooth Prime, which appears at the Eastern segment of the Auric Basin once the pylons have been activated, is an absolute nightmare without exceptional coordination. First off, it has high-damage attacks that mercilessly punish players for grouping together in one place to stack buffs while fighting it. Second, it will periodically try to eat a downed player, giving others an incredibly small time window to break its (considerable) Defiance before it finishes and gets a permanent, stacking damage boost - pulling this off requires almost every player in the arena to equip stunning/dazing/launching abilities and weapons and keep them just for this moment. Finally, its health scales up the more people there are fighting it, and many might come in not realising what's needed for this fight, so the more people there are, the harder it is to kill it in time.
• The final story boss for Living Story Season 3 chapter "Head of the Snake" is Bloodstone-Crazed Caudecus. When that chapter was newly released, the boss fight was very aggravating. You fight in a cramped circular room with him at the center. He throws so much area-of-effect attacks that push you, and occasionally he will trigger Chaos Magic which you must defend with - with very little window. The Chaos Magic shield you cast was supposed to stun him, but because attacks come out as he gets stunned, you're otherwise knocked away to take the opportunity. It doesn't help that he summons a creature to fight with him as his health goes down - one of which is a Veteran Jade Archer. As mentioned earlier, Veteran Jades are annoying to fight with because of their mandatory invulnerability phase and always crowd-control attacks. A Veteran Jade Archer launches ranged attacks that knock you back in addition to the ones attacking right now. The whole thing was pretty much stressing many players with characters generally at a major disadvantage (apparently Guardians and Necromancers had a slightly easier time). A day after, a patch was released that thankfully toned down the fight - with longer window of opportunity to cast the shield, a wider room, less attacks, and the Veteran Jade Archer replaced with a more manageable enemy.
• The Eater of Souls in the first chapter of the third Act, The Departing. This boss doesn't do an incredible amount of damage. Actually, it's not that hard to actually tank the damage and do damage, it's it's self-healing that makes it difficult. This guy has the capability to knock you down before sucking the life out of your ghostly corpse and heal itself back to 100%. This wouldn't be so bad, if it wasn't for the fact that this little abomination can use this skill every twenty seconds. It requires precise timing to be able to stun it before it can start siphoning your health. You're better off just running away when it tries to do it, but it takes some time to be able to understand an get used to the rhythm. Your friends in the instance ends up being fodder for the creature's health and they can't even do anything besides giving you mediocre boons. Most folks ended up with a less than amazing revival, when they are brought back to life naked from their armor being rekt in the afterlife.