Phase 1, roughly half of the raid will stay in the arena, dealing with waves of monsters. Leave your AoE classes and those who can interrupt here. Interrupt heals, dispel HoT's and make sure you focus-burn the Vrykuls. The other half of the raid will go through the tunnel, where they will need to defeat two bosses and force Thorim to enter the arena. The first boss will blast either side of the room on even periods of time - The second mini boss should not pose threat if you engage him quickly - just make sure you get away from anyone charged with Runic Detonation. After you defeat him, run to Thorim and hit him. Phase 2 will begin. Casters and Healers should spread out, tanks should rotate to avoid anyone being hit while with Unbalancing Strike, and everybody in the raid should get away from the lightning fields Thorim creates between himself and any point on the walls.