The fight has 2 phases. You must clear a corridor/gauntlet to pull Thorim into the main room to start P2.
In P1, split your raid in half. While the corridor group that should mostly consist of AoE heavy specs immediately enters the corridor on the left, the other half of the raid stays in the main room.
Be careful in the corridor/gauntlet, the last section is trapped, don't walk in the middle.
In P2, you fight Thorim himself and don't have to deal with adds anymore.
The Hard Mode for this boss is relatively easy, and is enabled by completing the corridor/gauntlet in less than 2:45 minutes.
Sif will be casting different abilities on the raid in P2, but it won't make a massive difference honestly.
Split your raid in half, with AoE heavy specs going in the corridor. Interrupt the Evokers and Acolytes. Do not run in the middle of the room leading up to Thorim at the end of the corridor.
CASTERS
BE SPREAD TO PREVENT
CAST SPEED REDUCTION
EVERYONE
WATCH HIS ARM
Activate a pillar in the room that will frequently deal light damage to a nearby player.
Activate a pillar in the room that will frequently deal light damage to a nearby player.
Thorim is now active and can be killed. Don't stack and rather spread your raid in stacks of 4 to 5 players around the room.
EVERYONE
DON'T STAND NEAR THE LIGHTNING
TO DODGE THE INCOMING AOE
EVERYONE
DON'T STAND IN THE BLIZZARD
Damage bouncing on nearby players, inflicting 50% more damage for each targets it touches.
This is the reason why you spread in small groups, if it bounces too much, it will end up one shotting players.
An AoE in the room inflicting moderate damage and slowing people.
Damage on the whole raid, inflicting constant pressure.
Consider using a Frost Resistance Aura.
Damage bouncing on nearby players, inflicting 50% more damage for each targets it touches.
This is the reason why you spread in small groups, if it bounces too much, it will end up one shotting players.
Heavy hitting tank ability that reduces defense. Swap tank after each.
An AoE in the room inflicting moderate damage and slowing people.
Damage on the whole raid, inflicting constant pressure.
Consider using a Frost Resistance Aura.