Out of all the forum posts written concerning raid content back in Burning Crusade, I remain fondest of a guide to Tier 6 written by Gragnarth of Andorhal, who I assume must be a deeply cynical person by nature. Within you'll find expert tips on Illidan ("If YOU get demoned you say something to the effect hey I got demoned, and then hope that you are well liked"), Shade of Akama ("I play a fury warrior, and as a result i have no clue what the strategy for this boss is"), and Rage Winterchill ("Make sure you have at least 1 person bandaging the Main Tank every minute"). But the comment that seemed to get the most mileage was one concerning Illidari Council, which was colorfully described as "THE SUPER BOWL OF NOT STANDING IN THINGS!" This phrase subsequently entered the parlance of many a raiding guild, and I'm reminded of it whenever I look at AoE-intensive fights.
Kel'Thuzad isn't a fight with the kind of AoE damage you'll see on (for example) Malygos, but I rather like to think of him as being the Super Bowl of spreading out. For every time you've heard your raid leader howl at the raid to "Spread the ^*#% out!" before, you'll be hearing it five times more here, and with good reason.
As a quick note, most of the Wowhead links in this article are somewhat outdated and represent the damage done by abilities Kel'Thuzad previously had in 40-man Naxxramas. I include them here because Wowhead commentary is often helpful, and I expect the information to be updated soon.
Kel'Thuzad has three phases, although for most members of the raid there's very little difference between Phases 2 and 3. Phase 1 is very simple. Phases 2 and 3, by contrast, comprise an endurance battle of sorts with a soft enrage in the form of increasingly unhealable damage from summoned adds.
Before you start the fight, you'll want a minute or two in the hallway before Kel'Thuzad's room to plan which parts of your raid are going where. As previously stated, everyone (barring melee) needs to spread out with a minimum distance of 10 yards between each other while staying within range of the heal team. Kel'Thuzad is typically tanked in the middle of the room with the rest of the raid fanning out in concentric circles.
Phase 1 is very straightforward; Kel'Thuzad himself is immune to all damage, and you'll be killing the adds he summons. Park your raid on the big green circle directly in front of him, and shoot/nuke the hell out of the adds, who will be approaching from all sides of the room. With competent DPS, Soldiers (skeletons) and Soul Weavers (banshees) will all be killed by ranged DPS well before they reach the raid. Unstoppable Abominations will need to be tanked, and melee should burn them down as quickly as possible to prevent Mortal Wound from stacking too high.
Healing on this phase is generally light, and both caster DPS and healers should conserve mana. Phase One lasts for 3 minutes and 48 seconds on both normal and heroic, and then Kel'Thuzad will become active and begin Phase Two.
Frostbolt: This comes in two varieties, the single-target and multiple-target. Interrupting the single-target Frostbolt (which has a cast time of 2 seconds; make sure that enemy cast bars are enabled on the default UI or through mods) is of paramount importance due to the high damage it causes to the tank. Melee should set up a rotation in order to ensure that it's always interrupted. The multiple-target Frostbolt is an instant-cast ability hitting all raid members and just has to be healed through. This will occur every 15 seconds, but on the plus side it causes significantly less damage than the single-target Frostbolt.
Mana Detonation: Periodically Kel'Thuzad will place an unremovable debuff on a mana-using player that will cause them to explode in 5 seconds. Interestingly, it doesn't cause any damage to the debuffed player, but does to anyone within 10 yards. The amount of damage caused is a function of the size of the target's mana bar, with larger mana bars doing more damage than smaller ones; suffice it to say you do not want to be within range of a Mana Detonated healer. Casters, Hunters, and healers can avoid splash damage at all times by maintaining range from each other. The worst person it can go to is an Enhancement Shaman or Retribution Paladin in the thick of melee around Kel'Thuzad. If you are one such player and you get Mana Detonation, run like hell to an unoccupied area before you explode (and during "prep-time" in the hallway before KT, you'll want to designate one such area).
- Shadow Fissure: A very obvious red AoE will randomly appear around a player and explode after 5 seconds, killing anyone within it. Solution (as always) is to run out before that happens.
- Frost Blast: This is what you're most likely to die from, and why it's particularly important to maintain 10 yards from other players at all times. Frost Blast instantly encases the target (and anyone within 10 yards of the target, and anyone within 10 yards of that target, and so on and so forth) in a block of ice draining 104% of their health over 4 seconds. They must be healed as quickly as possible. The key to handling this as a healer is not to panic and attempt to top the target/s off; they do not need to be healed to full in 4 seconds, they just need enough health fed to them to survive the duration. With that said, this can be a truly ugly thing to heal if it goes to a melee DPS as it invariably chains. If you have a lot of melee, you'll want to split them into two groups standing on top of each other, one on either side of Kel'Thuzad, to minimize the potential damage. As a note, the tank can get Frost Blasted on 25-man but can't on normal.
- Chains of Kel'Thuzad: Ah, the beloved Mind Control ability. You won't have to deal with this on normal, but it's a charming part of the 25-man experience. Kel'Thuzad will mind-control up to 3 people for 20 seconds, and they must be crowd-controlled as quickly as possible as their damage and healing are vastly increased for the duration. Unless stopped, they will heal and buff Kel'Thuzad in addition to causing damage to the raid. However, they are completely susceptible to all manner of player crowd-control spells -- stuns, roots, Fear, polymorph, silences, etc. -- so use whatever CC spells are available in your raid and make sure they don't get loose. Even if this means that a crowd-controller has to stop DPS entirely, it's worth it -- Kel'Thuzad tends to be a long fight, and losing time or raid members to unwanted heals or damage is especially catastrophic while the raid is still gearing up. There is a 1-minute cooldown on the Chains ability.
There's no real strategy to Phase 2 on Kel'Thuzad; it's a fight that hinges completely on personal responsibility on the part of each raid member. People need to maintain range from each other at all times, move out of Shadow Fissures, survive Frost Blast, crowd-control any Mind-Controlled targets as necessary, and whittle Kel'Thuzad's health down to 45%, at which point Phase 3 will begin.
Phase 3 is actually the same as Phase 2 in all respects barring one:
Guardians of Icecrown: Kel'Thuzad, realizing his number may be up, begs the Lich King for assistance, which arrives in the form of 2 crypt lords on normal and 4 on heroic. You'll need one offtank on normal and 2 or 3 on heroic depending on raid makeup. The Guardians can spawn from any of the room's seven sub-chambers and will do so within roughly 5 seconds of each other, so offtanks will need to be alert and may want to use /target macros. The Guardians have no magical abilities and their melee is very weak at first (to the point where it's not really a big deal if one spawns on the opposite side of the room from you and is loose for a moment, especially on normal), but their damage will increase by 15% every 15 seconds with the ability Blood Tap. This is essentially a soft enrage as the offtanks will eventually start taking massive and increasingly unhealable damage.
Tanks: If you're main-tanking Kel'Thuzad, you'll probably want to wear your high-stamina set in the event that a Frostbolt doesn't get interrupted. The ability can't be Spell Reflected, but the main tank position is still prejudiced towards a Warrior or Death Knight tank due to the relatively short cooldowns on their interrupt abilities. Offtanks can DPS through Phase 2 if they want to, but we generally don't bother with this as it just means more people who have to be healed in the event that Frost Blast goes to a melee DPS. If you're not going to DPS, you'll want to stay on the otuside of the raid toward the sub-chambers where the Guardians will eventually spawn. Spend Phase 2 avoiding Shadow Fissures and reading something to better yourself. My current pick is Herodotus' The Histories. If you get through more than one chapter, send a strongly-worded letter to your DPS.
As a side note, Kel'Thuzad is tauntable if all hell breaks loose and the MT goes down.
DPS: Save all of your serious cooldowns, Heroism/Bloodlust, and trinkets for Phase 3 -- at this point it's of paramount importance that Kel'Thuzad dies as quickly as possible. Casters and Hunters need to be careful with their mana usage in Phase 1, as KT even in full Tier 7/Tier 7.5 gear is a relatively lengthy fight, and the importance of spreading out can't be overstressed.
If you're doing the 25-man version of Kel'Thuzad, certain people will need to be designated as crowd-controllers to lock down any mind-controlled players. Mages are generally best at it -- there's still not much that can beat Polymorph -- but you'll need back-ups in the event that they get MC'd or you just don't have enough Mages. As soon as Kel'Thuzad casts Chains, CCing the affected players matters more than anything else you do.
Interrupts on the single-target Frostbolt are also hugely important as they represent a source of very high and otherwise unavoidable damage on the tank. This is generally a job for melee, who will want to have a specific rotation worked out before the fight begins. Mistakes can and will happen, but you really can't allow the tank to absorb this at all, especially as the fight goes on -- the healing load is just too high.
Some people recommend that melee DPS wear a piece or two of their epic frost resist gear to increase the chance of resisting Frost Blast. There's certainly a valid point here as healing multiple Frost Blasts is easily the most difficult part of the fight for the heal team, and you may want to do this even with a well-geared raid if most of your DPS are melee. Otherwise, with a lot of melee you'll want to keep them in two groups on either side of Kel'Thuzad and at maximum melee range at all times to avoid the possibility of Frost Blast also chaining to the tank.
Healers: The fight starts off easily and then skyrockets in difficulty. High mp/5 or Spirit gear here will be useful. Priests and Druids will want to have their Circle of Healing and Wild Growth abilities off cooldown as much as possible to prepare for Frost Blast hitting the melee. Conserve mana through Phase 1 and try not to blow through too much in Phase 2, because Kel'Thuzad still has about half of his health left to go at the beginning of Phase 3. It's at this point that the heal team will start firing on all cylinders to keep up with multiple tanks, steadily rising damage from the Guardians, the constant raid-wide damage from the multi-target Frostbolt, and the ever-present need to use quick heals or cooldowns on Frost Blast. DPS absolutely must interrupt the single-target Frostbolt in this period, exercise complete control over mind-controlled players, and should be prepared to use healthstones/potions/bandages if necessary.
Kel'Thuzad is the subject of the achievement Just Can't Get Enough (the same on both normal and heroic), which will require you to pull and kill 18 abominations during Phase 1. With good DPS, this isn't actually too tough, but you will need to actively pull that many in order to get 18 (otherwise you'll get 6-8 during an average Phase 1). Start pulling them as early as possible because Phase 1 lasts less than four minutes, and ideally you want them all down before Phase 2 starts. You'll find that deliberately pulling an abomination will usually cause 2 other aboms to aggro at the same time, which speeds the process considerably.
10-MAN vs. 25-MAN
This is one of the few encounters that's substantially different between the 10-man and 25-man versions. Normal-mode Kel'Thuzad can't mind control, Frost Blast the tank, or spawn more than 2 Guardians of Icecrown. Otherwise, the usual stipulations apply -- KT has less damage and health on heroic. Between that and the fact that most potential damage on the KT fight (Shadow Fissure, splash from Mana Detonation and chaining Frost Blast targets) is entirely avoidable, the fight is significantly easier to deal with on the 10-man version.