At the start of the fight when you engage the Jormungar Behemoth and the other adds, Thorim will cast Sheath of Lightning on himself, making him almost entirely invulnerable to attacks. Have one tank turn the Jormungar away from the raid, and have the rest tank and AoE the adds, focusing primarily on the healers (the smallest one in front and the dark runed dwarf in the back).
After the initial mobs are down, the lever next to the gate on the left will become active. The raid should now split in half; one half should go down the hallway to reach Thorim, and the other half needs to stay and fight off adds and attacks by Thorim. At this point you will want to have both fire and nature resistance. You will have 5 minutes to complete the gauntlet (getting to Thorim) before he enrages and wipes the raid.
Once Thorim is activated, he will proceed with the fight, progressively gaining damage and attack speed as the fight goes on. This is a passive enrage timer, so speed is not a dependent requirement on his tank phase, but it is helpful.
The Gauntlet
The gauntlet requires a minimum of a tank, a DPS, and a healer. It is best to have additional DPS (ranged with CC preferred), as much as you can spare from the group killing adds, to quickly reach the end of the gauntlet where Thorim will engage the party. Speed is key at this point, because after five minutes he will wipe the raid. It is also important because everyone in the raid will be taking a lot of damage and healing becomes an issue during the encounter with Thorim.
There are 3 groups of 3 Warbringers. They can be polymorphed, feared, and mind controlled and other forms of crowd control. Two should be crowd controlled while the raid kills the others. At the end of the hallway, there is a large golem. It raises one arm and does damage to anyone on that side of the hallway, much like the Skadi encounter in Utgarde Pinnacle. A short time after it is engaged he will gain a buff that decreases his damage by 50%, so if using cooldowns it's best to use them early.
After defeating the golem the door behind it will open and mobs will begin to spawn from the top of the stairs. Defeat the two sets of two mobs already standing there and make your way to the top of the stairs. When the giant is pulled he will buff any adds still alive, but the adds will stop spawning. Make sure the healer is in the line of sight of the tank yet out of the giant's stomp. This can easily be accomplished by bringing the giant to the top of the stairs and having the healer about halfway down.
After the giant is defeated someone must cast an ability on Thorim to get him to enter the arena. Avoid the large circles on the ground as they are traps that will paralyze you for a brief time.
Entrance Room
Those staying in the room will need to stack up in the middle to avoid the charged orbs around the sides. This will also make area-of-effect healing to allies and area-of-effect damage to enemies easier to dish out. The Dark Rune Champions do a whirlwind and need to be kept away from the main raid, or disarmed. The Evokers casts heals-over-time that heal for massive amounts and also cast magic damage shield that can be stolen by mages. Both are dispellable and need to be dispelled quickly. Wound poison also help here to reduce the amount they heal. But make sure to spread out when he targets someone with a stormhammer.
Alternatively, you can stay bunched up for stormhammers as damage taken during this phase is light unless someone gets too close to an overcharged orb. If your group chooses this method, it is optimal to make sure the healer with the center group has instant cast and group heals.
Boss fight
Once the gauntlet group reaches Thorim he will jump down, he needs to be picked up by the MT immediately. About 15 seconds in he will do his first Unbalancing strike, when he does this, the OT needs to taunt immediately. When you see the little lightning balls coming from a column this means he is about to use lightning charge, and that side of the room in a 60 degree angle around the column wil be shocked for a very large amount of damage, one shotting players after the third one. Spread out around him to avoid chain lightning links as well.
Lose your Illusion
If your group reaches Thorim within 2:45 (6:15 on Heroic) of initiating the encounter, Yogg-Saron under the illusory guise of Sif will join the battle. This changes several things about the encounter greatly increasing the difficulty.
Thorim will have 30% more HP and 40% more damage, while the damage is not a large deal, the HP will mean he will gain considerably more stacks of Lightning Charged meaning cooldowns will be required to prevent tank death towards the end. Thorim will gain about one buff per 10 seconds. Provided that the healers use their cooldowns effectively from buff#7 and on, the damage thorim will deal to the MTs will become very hard to heal once he gets past buff#12 and it's almost guaranteed to one shot tanks once he gets to buff#15. The healers and the tanks must pre-arrange their cooldowns and abilities in an orderly and well-timed manner so as to ensure maximum survivability.
Thorim's Chain lightning will be very dangerous to melee on Heroic mode so it is advisable to form 2-3 melee camps more than 8 yards from each other around the boss, as with Kel'Thuzad at 60.
Raid Positioning
Sif herself is unattackable. She has four abilities which when chained together can outright kill a raid member from full to dead on Heroic mode.
* Frostbolt Single: This does 11,000-13,000 Frost damage and is chain cast on random targets when Sif isn't casting anything else.
* Frostbolt Volley: This is the same damage as her single target frostbolt and hits everyone in the room every 10-15 seconds.
* Frost Nova: Sif will often teleport to a random location around the room and begin casting Frost Nova which takes 2.5 seconds to cast. It does extremely high amounts of damage (13,000-15,000) so avoiding it by running away is paramount but not always possible with the snares in the encounter.
* Blizzard: Much like Shade of Aran Sif will conjure a Blizzard which orbits the centre of the room, making the centre 'safe'. It does 4,500 Frost damage every 2 seconds and snares targets by 50%.
Due to the extreme lethality of Sif's abilities on Heroic mode one piece of frost resist gear and frost resist aura is advisable to avoid raid members being killed outright from full to dead within the space of half a second. More than one piece is not reccomended as it will hinder your DPS too severely.
When Thorim is defeated 'Sif' will be revealed for what it is: an influence tentacle of Yogg-Saron.
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