25-player normal mode abilities
Lord Jaraxxus has 20.2 million hit points.
Jaraxxus will often buff himself with this ability, which increases magic damage done by 20% per stack. Starts with 10 stacks and thus buffs Jaraxxus' damage by 200%. This can be spellstolen and magic dispelled. If Jaraxxus is allowed to cast with this buff active someone will assuredly die.
Inflicts 24375 to 25625 Fire damage and an additional 9263 to 9737 Fire damage every 1 sec for 5 sec. This is always cast on his primary aggro target. Two second cast, interruptable, magic dispellable.
11700 to 12300 Fire damage to a random player and up to 4 additional nearby targets.
The Legion Flame inflicts 2925 to 3075 Fire damage every 1 second for six seconds to a random player. You also leave a new patch of Legion Flame on the ground every second which burns for the same amount of damage if stood in. Standing in one spot will eventually instantly kill you.
Burns the flesh off your bones! Absorbs the next 60000 healing received and decreases damage dealt by 50% for 12 sec. If Incinerate Flesh is not removed before it expires it will cause a Burning Inferno.
Burning Inferno inflicts 3900 to 4100 Fire damage every 1 sec for 5 sec to the entire raid. The Burning Inferno is caused by Incinerate Flesh not being removed before the duration expires.
Lord Jaraxxus opens a portal to the nether world, inflicting 9263 to 9737 Shadow damage to targets in a 10 yard radius of the portal and summoning a Mistress of Pain.
Lord Jaraxxus summons an infernal volcano which deals 9263 to 9737 Fire damage to nearby enemies and summons three Felflame Infernals.
Mistress of Pain
These have 1.5 million HP and melee about as hard as Jaraxxus himself.
Spinning Pain Spike
The Mistress of Pain spins away to a random target, inflicting 50% of their maximum HP in physical damage. She then returns to her primary aggro target.
These have about 400,000 HP and low melee damage. They are stunnable.
The Felflame Infernal shoots at a random target and begins chanelling this spell, which deals 5,000 Fire damage per second to nearby enemies.
Inflicts 6338 to 6662 Fire damage to their primary aggro target.
25-player heroic mode abilities
Touch of Jaraxxus
Inflicts 3900 to 4100 Shadow Damage every 1 sec on a random player. Inflicts Curse of the Nether on nearby players (but not on the targetted player).
Curse of the Nether
Inflicts 4388 to 4612 Shadow damage every 1 sec for 15 sec. Dispellable.
Nether Portal and Infernal Volcano are able to be targeted and must be destroyed to get the minimum of adds spawned.
Lord Jaraxxus is a fight with relatively simple mechanics with a moderate amount of healing required. Jaraxxus himself should be tanked in the center of the room, with ranged and healers set up in a wide circle. Spread healers evenly to avoid having large areas that are out of healing range. Fel Lightning, a chain lightning ability that increases in damage with each hop, can be very dangerous if the raid is not spread out. In order to reduce tank damage, Fel Fireball should be interrupted. Resistance Aura also helps reduce incoming damage.
Every 60 seconds, Jaraxxus alternately spawns a Nether Portal or a Fel Volcano. Nether Portals spawn a Mistress of Pain, while Volcanoes spawn 3 Felflame Infernals. Both types of adds must be tanked and nuked down by ranged as fast as possible. Melee should stay on the boss.
Jaraxxus will randomly cast Incinerate Flesh on one raid member. This debuff absorbs all healing on that target until a certain amount of healing has been done (10-man: 30,000; 10-heroic: 40,000; 25-man: 60,000; 25-heroic: 85,000). Healers need to react quickly: if the debuff wears off, the target blows up, causing a Burning Inferno to hit the entire raid for large amounts of damage over time.
The other debuff which Jaraxxus randomly deals to raid members is Legion Flames. This continuously spawns flames under its target for six seconds. The target should run away from other raid members (towards the room wall) while remaining in range of healers.
Lord Jaraxxus will also occasionally give himself 5 stacks (25-man: 10) of Nether Power that increases magical damage dealt by 20%, totaling 100% (25-man: 200%). Mages should Spellsteal this buff as it will increase their DPS and reduce Jaraxxus's damage. If there are too few mages to remove it efficiently, the debuff should be dispelled by priests, shamans, protection warriors and hunters (Tranquilizing Shot). In 25-man, more than 2 mages are needed: Spellsteal is very mana-intensive and triggers a global cooldown, so removing 5 stacks per mage will take a long time and nearly exhaust the mages' mana pools.
To do list:
Interrupt Fel Fireball casts
Spellsteal or dispel Nether Power
Tank and nuke the adds quickly (ranged only)
Healers: pay attention to Incinerate Flesh
Move away if affected by Legion Flames
Passive healing or damage reduction should be used as much as possible to make healer's lives easier. Use healthstones, health potions, bandages, etc. to get rid of Incinerate Flesh, and don't forget Drain Life, Bloodthirst and Rune Tap.
Heroic mode tactics (25-man)
The main difference between normal and Heroic mode is that the Nether Portals and Fel Volcanoes don't despawn. Instead, they will continue to spawn adds until the portals themselves are killed. Jaraxxus himself has a very forgiving enrage timer, but the portals are quite difficult to kill quickly enough. That means all DPS should prioritize portals and immediately switch to them when they appear. The portals will always spawn off Jaraxxus's left shoulder at 8 o'clock, about 10 yards away from him. Tanks should position the boss such that the portals will spawn approximately in the center of the room so that all DPS can quickly target them. All melee (except for Fel Fireball interrupters) should preemptively move to where the portals will spawn about 5 seconds beforehand. Once the portal is dead, melee can return to the boss.
If Jaraxxus is in the middle of casting Legion Flame or Incinerate Flesh on a raid member when it's time for a portal spawn, he may be turned at an odd angle and the portal might appear in an inconvenient spot. Everyone should be ready to dynamically adjust if this happens. Also, the timing of the Nether Power buff varies quite a bit and can cause a problem if its appearance overlaps with a portal spawn. Mages should be prepared to spellsteal the buff up until the moment a portal appears, at which time they should prioritize killing the portal.
The raid should aim to kill Nether Portals before more than one Mistress can spawn, and to kill Fel Volcanoes before more than three Infernals can emerge.
It is particularly important to avoid having multiple Mistresses, as they have gained a nasty new ability called Mistress's Kiss. The victim of this spell takes up to 14,350 shadow damage and is interrupted for eight seconds the next time he attempts to cast any spell with a cast time. The victim should cast a spell in a school he does not plan to use, so he is locked out of that school instead of his primary school. Players can avoid taking the damage while clearing the debuff if they cancel the spellcast before it goes off. Holy paladins, whose spells with cast times are all in the Holy school, are able to avoid getting interrupted by starting a cast and interrupting it right away (Aura Mastery used on conjunction with Concentration Aura does not prevent the interrupt). If multiple Mistresses spawn from the portal, shamans should blow Heroism/Bloodlust and melee should swap targets and help burn them down.
The infernals can also cause a few problems. They are difficult for the assigned tank to pick up, and their constant movement makes focus-fire on any given one problematic. They will frequently target a random player, fly across the room to their position, and begin AOE'ing. The infernal cannot be moved once it plants itself, so nearby players have to run out of range. If the infernal targets a melee player, the tank needs to move the boss to somewhere safe. Mages should take particular care not to get separated from the boss by the infernals - they should always remain close enough to the boss to spellsteal.
The adds don't have a large number of hit points, so it is usually easy to kill them before the next portal phase, and Jaraxxus has an extremely lax enrage timer. However, it will take at least 5 or 6 portal phases to kill Jaraxxus, so the main challenge of the fight becomes burning down the portals both quickly and consistently.
[info from http://www.wowwiki.com/Lord_Jaraxxus]