That One Boss/Role-Playing Game/World of Warcraft



""It would be unfair to make you fight all four Advisors at once, but...fair treatment was never shown to my people. I'm just returning the favor.""

- Kael'thas Sunstrider

This page is about bosses in World of Warcraft that give away grief like it's candy.


 * The original World of Warcraft
 * Arugal, the last boss of Shadowfang Keep, can immobilize your party then teleport away to attack them with Shadow Bolts, and can also turn one party member into a werewolf that attacks the others, making him exceptionally difficult for his level and considerably different from other bosses.
 * Archaedas, end boss of Uldaman, is a major step up in terms of complexity from anything previously encountered by players while leveling because of large numbers of adds that swarm the other players and heal the boss. There's also the fact that he was level 47 while the dungeon could be entered as early as level 38. The difficulty curve was tweaked in a later patch.
 * Majordomo Executus was considered a massively difficult encounter due to the need to coordinate the simultaneous tanking and crowd control of nine separate mobs -- this in addition to the harsh requirements to summon him in the first place.
 * Razorgore, the first boss of Blackwing Lair, requires tanks to pick up and kite potentially dozens of adds before the boss can even be fought, while healers do everything in their power not to attract attention. Many Molten Core/Onyxia level guilds simply folded on this fight.
 * At higher levels, a new element of difficulty comes into play; it's possible for people who don't know the fight to kill him (which is fairly easy, as he has less than 500,000 HP) before destroying the eggs, which results in him wiping the raid.
 * Immediately after Razorgore comes Vaelastrasz, an absolute balls-to-the-wall slugfest on a short timer where one mistake with aggro can wipe the raid. Vael was renowned as a guild killer back in the day. Keep in mind that Razorgore and Vaelastrasz were not simply the first two bosses in Blackwing Lair--they were the first two encounters, as well as the two hardest bosses in the instance (only Nefarian himself comes close)! This is a textbook example of how not to build a dungeon, because while Blackwing Lair was very impressive, most players never got a chance to see it because of Razorgore and Vael.
 * Another boss is Broodlord Lashlayer: If your tanks weren't in essentially the very best gear possible, they would probably die to Mortal Strike because it could crit for 8k on plate. You'd likely need multiple tanks with great gear because Broodlord uses a knockback that reduces the target's total threat by 50%. If you're melee and you ripped aggro, prepare to die. If you're ranged and you ripped aggro, you better run in and die fast otherwise Broodlord will cleave and kill your ranged/healers. Then if you wipe, have fun reclearing the Suppression Room; easily some of the worst trash in the whole game.
 * And the Suppression Field reduces your movement to 1/4 of normal and your attack and casting speed to 1/5 of normal. Walking through is a chore and fighting is extraordinarily difficult.
 * Ayamiss the Hunter deserves special mention due to the tendency of players not familiar with the fight to accidentally aggro him before the room was cleared of trash or the raid prepared.
 * Dr. Weavil. I'm not sure if you can fight him anymore, but he was absolutely hellish. If you wanted a chance at getting the Black Qiraji Battle Tank, you needed to fight him. He employed a permanent mind control, and a powerful multi-target nuke with a 2 second stun. You needed at least 10 people to fight him, and those 10 people would also be trying to ring that gong at the end first.. The whole chain was full of these. Maws, Lord Lakmaeran (Who only one person could loot) and his many chimeraoks.. It was worth it in the end, though. The mount is pretty awesome.
 * You can fight him but you can't get the Black Qiraji mount anymore, removing virtually all the incentive to kill him.
 * Princess Huhuran is a gear-check boss, requiring insane levels of Nature resistance and near-perfect timing. Any less and you might as well not even attempt her.
 * Immediately after Huhuran in Ahn'Qiraj comes the Twin Emperors, a sheer coordination and teamwork fight that can still frustrate players two expansions later, due to the fact that without precise tanking they simply heal each other to full within seconds.
 * C'thun was brutally hard in his initial version, mainly due to the fact that he would cast his raid-wiping Eye Beam attack without any delay for players to enter the room. He was actually so difficult that players came to the conclusion that the fight was literally impossible to win; unbeatable at the current level of gear. Whether this was true or not, Blizzard eventually nerfed the encounter and the first kills came hours later. However, even at 80, if someone jumps the gun, the raid will likely wipe if he does Eye Beam.
 * C'Thun remains the only raid boss to be completely un-beaten prior to any sort of nerf (legitimately at least: one guild did kill him, but it was by using an exploit that allowed them to basically walk through the walls in the room).
 * Old Naxxramas deserves its title as the final and most difficult raid dungeon of classic WoW. It would be easier to list the bosses that are not That One Boss, but some standouts are listed below.
 * Patchwerk is a gear check boss that requires perfect timing from the healers to avoid having the tanks die.
 * Gothik the Harvester is easy once you get to fight him. The trouble is the dozens of creatures he summons to attack you first.
 * Anub'Rekhan would summon adds from dead players in addition to the dead adds. Que exponential hell.
 * Thaddius is pretty bad for groups that can't figure out how to move properly on his Polarity Shift, especially since each death makes it significantly harder to beat his berserk timer. Not to mention the mental defectives who need Slow Fall every time just to make that damn jump from the minibosses' platform to Thaddius.
 * Heigan the Unclean has a unique mechanic requiring players to move across a floor in sync, chasing the "safe spot" amid lethal eruptions. The so-called "Heigan Dance" claims far more lives than the boss himself, who is notably easy. In particular, it's extremely sensitive to the latency of your connection to the game, meaning players who disconnect or lag behind others are pretty much doomed. Even the most hardcore guilds out there wouldn't yell at you for dying during Heigan the Unclean. (Heck, they won't even yell at you now, when there's an achievement for completing Naxxramas without anyone dying...and another for defeating Heigan without anyone dying!) if you lag. It used to be a slogan that lag was a very passable reason for dying against Heigan...heck, you can even lag a tad and die! Another thing that made Heigan bad was that in order to see the particles, you had to turn the graphics settings up. Not a good idea for some people; given that part of the draw of the game is that you can run it on a toaster. In addition to server and clientside lag, you could lag running the particles.
 * The Four Horsemen had the strictest timing and movement requirements of any boss before or since, needing as many as eight tanks to perform correctly. The few guilds that successfully defeated this encounter had no trouble at all with the remaining bosses, and was widely considered the hardest boss(es) to kill until the instance was retuned. They still are an incredibly complex positioning affair, and PUGs tend to not understand where to tank them or how to switch off, thus resulting in people getting two marks at once or being AO Ed to death because no one's close enough to the two in the back The whole thing was made worse by the fact that the tanks had to have their taunts land on every single "rotation". And that taunt relied on "Spell hit". It was not uncommon for high end guild to "pinch" tanks with 4 pieces of Tier 3 gear (which improved the chance for a taunt to land) from other high end guilds, causing a fair bit of friction within the community, as this was before you could pay to transfer your character to a different realm.
 * Gluth can devour his adds, which heals him and makes victory impossible for those who don't know how to kill them.
 * Frogger isn't exactly a boss, but enough players have died trying to avoid the moving slimes that it has become somewhat notorious.
 * Although it has gotten to the point where people are so tired of people dying to Frogger during the walk back to Those Three Bosses after it (seriously, Grobbulus, Gluth and Thaddius are all after it, all guaranteeing lots of wipes) that they simply have someone ankh and resurrect everyone individually...
 * The Burning Crusade
 * Warbringer Omogg in Shattered Halls is a two-headed ogre that occasionally chooses to attack a random player other than the tank, and can't be taunted. Bring a bunch of cloth-wearers in there and prepare to get squished.
 * Kael'thas Sunstrider, in Magister's Terrace. Sure he is a Degraded Boss from his former status as the Final Boss of Tempest Keep, but is still the hardest 5 man boss in Burning Crusade, due to the massive magical damage he inflicts.
 * The Shade of Aran is an atypical boss in that he has no physical attacks by default, but casts devastating spells on random targets. This turns the fight into a movement and healing struggle that becomes absurdly more difficult when he summons elementals to protect him at 40% health. While all of his spells could be interrupted, locking out all three of his magic schools (Frost, Fire, Arcane) caused him to whip out his staff and unleash a devestatingly powerful melee attack (at the time around 10k, enough to one-shot almost anyone it hit). Also, I will not move when flame wreath is cast, or the raid blows up!
 * Netherspite (technically a Bonus Boss, although this could be said of at least half the bosses in Karazhan), is of the "one stupid mistake can wipe the raid" variety. His mechanics require rotating players through a series of beams, a mistake with any one of which can result in the wrong person tanking or taking damage and completely wreck the plan.
 * Lady Vashj in Serpentshrine Cavern is not especially difficult in her first and third phases, but her second phase requires players to kill Tainted Elementals that stay in one place at the edge of the arena and only appear for 15 seconds before despawning, take the Tainted Cores (which prevent the holder from moving), and throw them from raid member to raid member until someone can use them on one of her four shield generators. To make matters worse, Mooks are coming in; the Enchanted elementals allow Vashj to AOE the raid if they reach her, and the striders Fear anyone who gets close. If your group isn't coordinated, you have no chance of making it through this phase.
 * Kael'thas Sunstrider in Tempest Keep is a five phase boss that can last upwards of fifteen minutes. Assuming you survive his advisors, each of which is a miniboss in its own right, you then have to deal with his attacks, including Mind Control, a Pyroblast that will instantly kill anyone it hits, and a phase where everyone is flung up into the air and must "swim" to avoid deadly floating orbs.
 * Teron Gorefiend was of roughly average difficulty for the Black Temple when it was new, when level 70 was the maximum and people went into the Black Temple expecting a challenging raid. However, go in there at 80 and he becomes That One Boss. The Black Temple was designed for 25 players of level 70, but at level 80 everyone has much higher values in every stat, so a group of even 10 or 12 level 80s can steamroll through the place as long as at least a couple people know what they're doing. Teron Gorefiend, however, requires using new temporary abilities quickly, so if people go into Gorefiend's room without knowing the plan, they will wipe.
 * Of course, it's possible to bypass the ghost phase entirely with enough DPS (roughly 42,000; difficult for early Lich King raids but easy for Cataclysm raids).
 * Reliquary of Souls is done in three phases, each of which has completely different mechanics. In one phase, you can't heal; in the next, you can't regenerate mana, and in the third, all damage you inflict is reflected back onto yourself.
 * Illidari Council is the one that gives PUGs the most trouble at level 80. Its rare that a group survives to council with more than 10 members, then there's invariably no mage (still incredibly handy for tanking purposes), a tank runs in without checking with the group, there's no caster interrupt with a short cooldown and the person assigned to interrupting instagibs themselves on reflective shield because they get bored. Definitely still requires coordination.
 * Groups with 80s can typically get past the other bosses (and possibly even Illidan himself) with 15-20 people, but they will ultimately need 25 to DPS the Council down and interrupt all the heals.
 * Kalecgos, the first boss of Sunwell Plateau, is nearly impossible for PU Gs to get past. He's nearly as long as the platform is wide, and the players can aggro him if they get too close to him (sometimes if they aren't even standing on the platform). This wouldn't be as much of a problem if he didn't lock everyone who isn't on the platform out of the arena within seconds of being engaged. The fight requires killing Sarthrovarr and then bringing him down to 1% at around the same time; if you bring Kalecgos down too quickly, you lose because he stops creating portals to enable you to attack Sarthrovarr, and if you bring Sarthrovarr down too quickly, Kalecgos enrages. This becomes even more difficult if members don't take the portals to attack Sarthrovarr (which also can resultin "Kalec" the elf dying and you losing) or don't stop DPS on Kalecgos.
 * Brutallus, the second boss, was a gear check with a very strict enrage timer, and gained a reputation, in the words of one video guide maker, as "the cock block of Sunwell".
 * Felmyst, who's only the third boss out of six at Sunwell Plateau, has a gas attack that can mind control raid members, and remains very difficult even for raids with level 80 players. Some players refuse to join Sunwell Plateau raids if they have not gotten past Felmyst.
 * M'uru was as far as many, many raids ever got. That boss ate entire guilds whole. And he wasn't even the final boss. Unlike some of the above bosses, though, M'uru did not scale as well against level 80 players as some of his peers. This has something to do with the fact that a) he has low HP in comparison and b) level 80s are very good at burning things down. Both make him considerably easier, since much of the difficulty lay in getting out the DPS to kill him in time.
 * Wrath of the Lich King
 * at the end of his quest chain in Zul'Drak. You have to control a Berserker to attack him while avoiding the barrels of blight he throws at you, and switch control immediately if you lose your Berserker. Not only is it difficult for those who don't use pets, but your character is actually a liability in the fight.
 * Ley-Guardian Eregos, in Oculus, is a unique fight in that it's conducted while mounted on drakes, using their abilities instead of your own. Once the mechanics are understood, it's simple, but try getting a pick-up group to learn it.
 * Loken, in the Halls of Lightning, was at one point statistically the deadliest boss in the game, accounting for more player deaths than any other. The fact that one of his attacks forces to you remain as close to him as possible, while another requires you run away as fast as possible to avoid instant sparkly death, may account for this. He didn't last very long, however, as better gear made his dreaded Lightning Nova survivable. With good enough gear to remove the threat of being killed instantly, the fight became so easy that a single character could beat this five man encounter at the appropriate level.
 * Compared to the other Keepers in Ulduar, Mimiron is a fairly difficult fight - it's long, has four phases each possessing a number of abilities that one-shot raiders, and has a tight enrage timer. In heroic mode, it's the same with higher damage, a five-minutes-shorter enrage timer - oh, and the room is on fire.
 * Yogg-Saron in Ulduar. If you touch a roaming cloud in Phase 1, you'll spawn an add. Do this enough, and even Kingslayer groups will wipe. Once the fight begins, every player gains a new debuff called "Sanity" that starts at 100%. During the fight, specific abilities will decrease your Sanity; if Sanity reaches zero, you become Insane--Yogg-Saron controls your mind and you begin killing the raid. Yogg-Saron is so powerful that he can break you and destroy your sanity. Phase 2 requires priority on killing tentacles, keeping Sanity as high as possible, and DPS entering portals to attack the Brain of Yogg-Saron. Phase 3 becomes more of a DPS fight, but still maintains Sanity. Every 12 seconds, Yogg-Saron casts Lunatic Gaze; staring into madness itself lowers your Sanity by 4% every second. This is easily countered by turning away; healers must never look at Yogg-Saron and tanks/DPS must quickly turn away. The Normal Mode version allows the four Keepers of Ulduar to assist your raid with various buffs. The Hard Mode versions reduce the number of Keepers as low as the raid wishes, even to, God forbid, zero Keepers for Alone in the Darkness.
 * Argent Confessor Paletress in the Trial of the Champion 5-man dungeon summons a giant shadow creature that casts an area of effect fear spell. Constantly. However, a great portion of the dungeon's difficulty is due to the fact that players treat it as a standard 5-man dungeon despite it being substantially more difficult than anything that came previously.
 * Devourer of Souls in the Forge of Souls is another recent addition that can be a nasty surprise for players used to the relatively easy launch dungeons. He has a hard hitting, fast spell that should be interrupted as often as possible, hard to see area effects and several nasty special abilities that require the whole group to react accordingly. One of them almost always kills the player it aims for, but fortunately its usually only used once at low health. Groups often die to Mirrored Soul, which results in the person being targeted taking damage when the Devourer of Souls does; if the healer dies, the rest of the group soon follows.
 * Keristraza in The Nexus. The party must constantly jump to prevent her Intense Cold debuff from stacking too high, but one of her abilities roots people to the ground. This is also made more difficult by the fact that it is often the first dragon boss that many players who don't raid fight, meaning people need to learn how to avoid her Tail Swipe and breath attacks while DP Sing, and the tank needs to turn those away from the rest of the party.
 * The Faction Champions encounter in the Trial of the Crusader raid dungeon is nasty for any group that isn't used to Player Versus Player combat. It consists of a randomly selected group of opposite faction characters who behave like PvP opponents: interrupting spells, using crowd control, and ganging up on individual raid members. The only way to beat them is to keep them crowd controlled while killing the healers, not as easy a task as it sounds.
 * It's also nearly impossible for pugs to complete because Faction Champions requires high levels of coordination. Pugs are not good at coordination.
 * The Beasts of Northrend require a considerable amount of cooperation from all the members, making it difficult for raids with many pick-up members to progress. The first opponent, Gormok the Impaler, throws Snowbolds onto people, which cause damage to players until the DP Sers kill them. In the second encounter, Acidmaw hits players with a poison that paralyzes them (before the debuff timer even expires), unless they run over to the tank on Dreadscale who has Burning Bile. The third opponent, Icehowl, is simpler, but if the raid wipes on one of them, they have to start over, and it is impossible to resurrect people between phases except with a Druid's Rebirth. PUGs are very bad idea since Acidmaw for the first half of his fight has a cone area of where his paralyzing poison hits and the said poison drains movement speed until it hits 0. Naturally the raid ends after everyone doesn't realize that they can just face him away from everyone and the kicker is that that was only the second part of the first encounter, you've still got 1 miniboss and 4 bosses left.
 * Icecrown Citadel, being the final major raid dungeon of Wrath, naturally has the most difficult encounters.
 * Lady Deathwhisper could have been considered this before her first phase was nerfed, as coordinating the killing of multiple waves of Mooks, some of which are immune to physical damage and some to magical, while dealing with her Mind Control ability, was tougher than even Blizzard intended. Even after the nerf, she's possibly the most difficult boss of the Storming the Citadel part of the raid.
 * Blood Queen Lana'Thel turns members of the raid into vampires. They get a damage buff, but soon have to bite someone within 10 seconds or get mind-controlled. Getting the damage output to beat her enrage timer involves working out an order of who should be bitten when, then finding the right person to bite before time's up. Losing one person to mind control almost inevitably means a wipe. With 25 people, this gets... interesting.
 * Professor Putricide remains the hardest wing boss thus far, with incredible raid coordination required to deal with all of his abilities, capped off with a severe time limit. Only five guilds managed to defeat him in Heroic mode prior to patch 3.3.3. It's also possibly the most annoying fight in the game for offtanks, who are isolated by being shoved into a vehicle that handles like a cow, where any of their tanking experience is useless, and are given a job where if they are slow on the mark even once the group is doomed, though it may take quite a while for that to become obvious to everyone else. And which the group can make vastly more difficult without even realizing. The fight is also a strenuous DPS race, and not only does he make adds that require the DPS' undivided attention, but in Phase 2, he throws out some of the most crippling damage-reducing debuffs in the game.
 * Sindragosa. While the Lich King himself is harder, he's the Final Boss and wouldn't go on this list. But Sindragosa? Oh, god. A fight that is basically built around RNG. Simply put, the mechanics of Sindragosa demand that everyone in your raid knows what their job is. Now that the Icecrown Citadel buff is up to 30%, any boss before Sindragosa can easily be killed even if people make a few mistakes. However, while learning the fight, you absolutely have to understand the mechanics; if not, you will wipe again, again, and then some more at the same point.
 * Cataclysm
 * Corla, Herald of Twilight, in Blackrock Caverns comes with two Zealots (three on heroic) who turn into powerful drakonids if you don't stand in the way of the beams that are causing them to evolve. The catch? Any party member who stands in them too long will be transformed and mind-controlled, and if someone makes that mistake, it will almost certainly result in a wipe.
 * Corla is one of those bosses that is the bane of PU Gs but a joke for coordinated groups, since what she requires players to do is actually extremely easy, but if one person messes it up once, she'll probably kill you. She also requires interrupts to stop her from fearing people through the beams, and while there's nothing stopping you from having a player not stand in any beams and dedicate themselves to interrupting her, in a PUG whether the assigned person will actually do it is always uncertain.
 * Erudax, the last boss of Grim Batol, after casting a large AOE effect that requires people to stack up on a single safe area, summons an add that can release more adds onto the players, and if not killed quickly enough, heals Erudax for 20 percent of his maximum health. On Heroic Mode, there are two of those adds.
 * Ozruk in the Stonecore paralyzes people before launching a powerful attack that can one-shot non-tanks, and the only way to remove the paralysis in time is to get a damage over time spell on you. This is simple for melee classes, as attacking Ozruk causes damage, but ranged classes must reflect their spells off him when he has Elementium Bulwark up so they can get a Damage over Time effect on themselves. Things get tricky for hunters, the only ranged class without spells. On heroic, a tank that understands the mechanics can solo him after the group has gotten themselves killed. For everybody else, he's That One Boss by way of Luck-Based Mission.
 * Admiral Ripsnarl in Deadmines summons many adds that use Cone of Cold on the party and eventually Coalesce and kill everyone in the blast radius if they're allowed to live for too long. Unfortunately, when he appears, people tend to ignore the adds and focus on him, largely because the adds don't do this in the non-heroic version. In that version, they just appear, whack feebly at random people for a bit, and then despawn harmlessly when Ripsnarl decides to reappear.
 * Vanessa Van Cleef in Deadmines; she is only available on Heroic, but is required for completing the dungeon on that difficulty. To even reach her, you must work your way through a room full of fire and collapsing icicles, fight the second boss without aggroing or being killed by the dozens of spiders on the ground, avoid spinning sparks that can kill you instantly and save several NPCs from attacking worgen. The encounter itself is relatively simple, but the detonating explosives (including one she detonates just as she's defeated) can kill you if you're not prepared.
 * Heroic Deadmines also features the Foe Reaver 5000, boss number 3 in the line-up. First up, one of the 3 dps must jump into a vehicle to manage adds throughout the fight who will wipe the group if they reach the party, and after 2-3 of them are coming at once it gets tricky at best to do so. Because of the adds, he needs to be pulled up the ramp and killed there, limiting the fight to narrow quarters, which makes it difficult to avoid his two attacks that will decimate anything near him (and one of them requires the group to haul ass away from a spot before he charges to it). Not fun.
 * Baron Ashbury in Shadowfang Keep. The fight isn't particularly healing intensive nor tanking intensive. However, the fight rests almost solely on your DPS to interrupt Ashbury's heals with Stay of Execution and Mend Rotting Flesh. If your DPS doesn't interrupt, you will eventually wipe. This fight becomes a god-awful nightmare in pugs because most pug dps are just not capable of interrupting spells. Thankfully, his worst ability, Mend Rotten Flesh, has been removed. Since he would often cast it immediately after Stay of Execution, and Pain and Suffering can't be ignored, either, victory was impossible without three people who could interrupt on a regular basis, and many groups only had two people who had the capacity to do so, meaning that they would likely fail regardless of their players' skill at interrupting.
 * Lord Godfrey is only moderately difficult as a Heroic boss at level 85, but when players go through normal Shadowfang Keep in their late teens and 20s, his abilities (summoning adds, placing a DOT on random people with Cursed Bullet, and a frontal cone AOE that almost certainly kills those standing in it) are considerably more complex than most other boss fights around that level, even compared to many of the other new bosses in Deadmines and Shadowfang Keep, who do not have their most deadly abilities in Normal. Cursed Bullet is especially difficult, since your party will likely be unable to interrupt or remove the curse like a level 85 group can.
 * Cursed Bullets should probably be singled out, because unlike other "mandatory dispel" mechanics in Cataclysm it is a curse, meaning paladin healers cannot dispel it at all, level 85 or otherwise. The encounter comes down to the tank and DPS interrupting the quick, frequent casts and dispelling the ones that go through...or forcing the paladin to heal through increasing shadow damage on multiple players. This is no mean feat when said inexperienced/lazy players are taking damage from the Pistol Barrage as well.
 * Jin'do the Godbreaker's first phase is quite easy, but in the second, players must trick Gurubashi Spirits into using Body Slam on the chains binding Hakkar while dealing with many adds. If a Body Slam doesn't hit a chain, it can easily mean a wipe.
 * Chimaeron in Blackwing Descent presents healers with a unique challenge. The players receive a buff that enables them to survive any attack while being reduced to 1 HP as long as they are at over 10,000 HP. The damage is so high during the fight that any more than this is wasteful. However, Chimaeron will frequently disable the buff, forcing the healers to restore the entire raid to full in order to survive. The fight comes down to their ability to quickly and efficiently maintain the raid at just over 10,000 HP in preparation for the burst healing phase, whereupon they pop mitigation cooldowns and heal like mad. While the fight is not especially hard for DPS, healer mistakes can easily lead to deaths, and a party with missing players cannot beat phase two (where everyone becomes immune to healing).
 * In Firelands, prior to the nerfs, Baleroc was one of these if you're a healer. His signature mechanic consists of putting a buff on the player who is tanking him, which increases his/her health exponentially. Healing players afflicted with Torment grants the healer a stacking buff, which turns into increased spellpower when the healer returns to the tank. The healer cannot increase their stacks any further while the healing buff is active, so they must coordinate with the other healers to make sure the tank is always being healed, the Tormented players are always being healed, and no healer's buff falls so far behind the tank's health stacking that healing becomes impossible.
 * Beth'tilac in the Firelands. While the DPS requirement is lenient enough that people can send the minimum of a healer and tank up top while everyone else stays down below, the adds are quite difficult. First, the Spinners can only be brought down with a taunt, and if left up, will do severe damage to everyone in the raid. Swarms of Spiderlings come out and must be AO Ed down, or else they will heal and buff the Drones, enabling them to go up top and shorten the first phases of the fight, giving you less time to kill the adds before Beth'tilac comes down. Another problem is that if the healer for the group up top goes up first, or the tank goes up too far in advance, one of them will die, Beth'tilac will then kill the other, then the rest of the raid will wipe from Beth'tilac's raid-wide attack once no one is up top.
 * The fight agains Ragnaros in the Firelands has three phases, and while the first phase is fairly easy, the second and third require good positioning to avoid wiping on the Searing Seeds, and quite a bit of movement when you're faced with the Engulfing Flame and Living Meteors. Perhaps the worst part of the fight comes in the transition phase, when eight Sons of Flame come out, and if one reaches the hammer, the group will most likely wipe. Not only does this require extensive coordination, but it's also something of a Luck-Based Mission, as the hammer landing in the center makes it more difficult to kill the adds in time.
 * Not to mention that HEROIC Ragnaros 25 man required you to run it with 3-4 healers(due to the RIDICULOUS DPS requirement), added another entirely new phase that was entirely based around RNG(who meteors would target, where roots would spawn, where fires spawn and where breaths would spawn) and if one member of your raid is out of position, you are royally screwed and wipe. Nobody on my server even got Ragnaros down before the nerf, and even after that it still took my guild quite some time before we got it down.
 * Echo of Tyrande in End Time can use many attacks that silence her enemies and are difficult to avoid. It's thus difficult to heal and avoid those projectiles at the same time, and if a healer gets silenced too often, it will result in a wipe.
 * Dragon Soul gives us the delightful Warlord Zon'ozz, who casts an enormous purple ping pong ball that must be bounced between targets before being rebounded to the boss. Bounces do AoE damage, which splits among targets and must be soaked. If the ball doesn't bounce enough times before returning to him, Zon'ozz cannot be killed before he enrages. If it hits the wall, he enrages instantly. He casts a damage over time debuff which, when dispelled, explodes and sends the target flying, but dispelling is mandatory due to the DoT. If the player's health is too low the explosion kills them, and if they are thrown into the ball they bounce it alone and everyone dies. Oh yeah, and the ball appears to move several yards forward after the game registers the bounce, it has a tendency to careen in a random direction (forcing the waiting raid members to dash over and catch it, or the tank to pull the boss over before it can hit a party member), and anyone caught in his frontal cone attack heals him. All this from either the second or third boss of the raid, and Hagara (#4) is laughably easy in comparison.
 * Yor'sahj the Unsleeping can be quite difficult for groups that aren't well coordinated. He sends out three slimes, only one of which can be killed (and it takes all the DPS to kill it before reaching him), which give him various buffs. Some of these buffs obviously are bad combimations, such as having green and red together (the former forces you to spread out while the latter forces you to stack), and some are clearly worse than others (especially purple, which makes people explode every fifth heal). A kill order must be worked out, and if he gets the wrong buffs or somehow gets all three, you'll likely wipe.
 * The real That One Bossitude of Yor'sahj comes from that the priority of slime killing will be completely different depending on the difficulty setting your raid team is using for the encounter. On LFR difficulty, getting rid of purple slimes is the most important, since one healer not knowing what they're doing can easily cause a total party wipe. However, green and red slimes at the same time is no big deal since green doesn't do significant splash damage on that difficulty level. On normal and heroic mode, however, the purple debuff isn't so bad (especially if paired with the blue slime, since you're not in danger of setting off the debuff if you don't have the mana to do much healing), and healers get used to healing around it, but combining red and green together would only work if you have a very melee heavy party. Get a raid leader in normal calling out slime priorities like it's LFR or vice versa and you're in for an exciting ride.
 * Nyxondra, a boss in an individual quest chain in the Badlands. Some genius at blizzard thought it'd be a very good idea to put her in a valley full of whelplings when you're with three vikings. The vikings have a tendency to aggro everything in sight, especially when they get feared. Who thought that was a good idea?
 * Ruuzel in Ashenvale is possibly the hardest low-level world mob in the game, that is, if you're a caster. If you use melee, she's nothing too difficult, but lord help you if you're a caster. You see, aside from just hitting you with her staff, her only two spells both interrupt casting, one of which is just a green Death Grip that she'll use if you get too far away from her, and the other is a melee with a knockback, that she will spam every 2-4 seconds. If you don't have access to too many spells with a cast time of less than two seconds, the fight is practically impossible.