This achievement is for the hard-mode version of the Thorim fight. You'll need to have your hallway group reach Thorim in 3 minutes or less in order to bring him into hard mode. At this point, Sif will join Thorim in the fight. Sif can't be killed, but she does cast the following spells at your raid:

• Frostbolt Volley
• Frost Nova
• Blizzard

In addition she removes Touch of Dominion. Thorim will do roughly 67% more damage and his health will be increased by 43%! The Blizzard spell effect will always be on the outside ring of the room, so be sure you don't tank in that area. Keep your melee spread out and be sure to dispel the frost novas!