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Lord Jaraxxus Group Setup
Before beginning the boss battle with Lord Jaraxxus you need to have clear assignments for your group. It is best to have the less geared of your tanks tanking the boss. Ideally this should be a Warrior or Death Knight that has reliable and frequent interrupts. Another melee DPS needs to also commit to being on the boss the entire time, like a Rogue, Warrior, or Death Knight. Together the Lord Jaraxxus tank and the assigned melee DPS need to interrupt all of the Fel Fireballs that Jaraxxus tries to cast.
You also need to designate a more AoE oriented tank to picking up adds. Throughout the fight a couple of different types of adds will spawn - Mistresses of Pain and Infernals. Mistresses of Pain can be tanked, but Infernals drop aggro frequently. The add tank should try to hold them as much as possible.
Healers need to be sure to have the entire room covered. The best practice is to mark healers and have them pick an area of the room that will be theirs. Raid members need to be sure to stay away from eachother throughout the fight, but especially stay away from the healers.
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As usual, speak to the guy by the door to start the boss fight. This intro is kind of long - Wilfred Fizzlebang comes out and tries to summon a Doomguard, and instead summons an Eredar Lord. Good times. Wilfred pays for that mistake with his life, but you've fought demons before. It's no big thing.
Tank Jaraxxus in the center of the room. The rest of the raid needs to give eachother a 10-15 yard space between them or his Fel Lightening will chain. The more people it hits, the more damage it does. It can very easily one shot someone if they're in the wrong place at the wrong time. Again, make sure that the tank and melee DPS is interrupting the Fel Fireballs. If they hit the tank, it will leave a heavy DoT on him. If one goes off, the DoT is a magic effect that can be removed by Priests and Paladins.
Also, throughout the fight Jaraxxus will buff himself with Nether Power, increasing his magic damage by 20% per stack of Nether Power. This is best spell-stolen by Mages, but you need to have a backup method for removing it if a Mage isn't present or dies. It can be removed with Purge or Dispell.
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Flesh from Bone!
Jaraxxus likes to burn things. As such, his two most dangerous abilities are called Incinerate Flesh (announced by screaming "Flesh from Bone!") and Legion Flames. Incinerate is a debuff that can be targeted on any raid member. Healers have 12 seconds to overheal until it fizzles, or else it will explode for 3-5k damage per second on everyone in the raid for 5 seconds. You have to heal through Incinerate Flesh until the debuff goes away. The amount that you have to heal varies by the raid size and difficulty. It is safe to say, though, that no healer should feel like they don't have to heal the person targeted because someone else will take care it. Everyone needs to be on it.
Legion Flames will also target a random raid member, and can be quite dangerous. The player targeted needs to run straight back to a wall and then kite along it. They will frequently explode with fel flames leaving behind a trail that no one should cross or stand in. This person will pretty much always require at least some healing or the flames will kill them, even if they're running as fast as they can. When Legion Flames ends that person can return to what they were doing, but the flame trail will remain for some time.
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Come Forth Sister, Your Master Calls!
As mentioned at the outset Jaraxxus likes to summon two different kinds of adds. The first he will call is a single Mistress of Pain, who will come through a demon portal. She's a pretty wicked add. She needs to be tanked, and all DPS needs to switch and kill her. Her main attack is something of a whirlwind, but not too incredibly dangerous. She also likes to use Spinning Pain Spike on a random target. When she does this, she'll grab them, leap up into the air, and slam them down, inflicting massive amounts of damage and messing up raid positioning. It isn't uncommon for a Pain Spike victim to get Fel Lightening chained onto them immediately after and die.
The second add type is Infernals. Jaraxxus will scream "Inferno!" and an Infernal Volcano will spawn in the room. The volcano itself deals AoE damage to anyone standing near it, but it also spews forth three Infernals which tend to just pick a person and follow them until taunted off. These infernals do basic melee attacks, but really like to stand still and do an AoE, or randomly charge someone leaving behind a trail of flame. Infernals don't have very high health, and move around too much for melee DPS to be very effective on them, so only ranged has to switch from DPSing the boss to killing the infernals.
It is worth pointing out that on Heroic modes the Mistress' Portal and the Volcanos will not stop spawning their respective adds until they're destroyed. Also, Mistresses gain a nasty spell lock ability that can totally shut down a healer and wipe the raid.