First, the 'tunnel group' must get to Thorim in under 3 minutes or Sif will despawn (this alone will get one achievement, gucci shoes but not the right one). This will make Sif fight bcbg shoes you with Thorim - this gives Thorim 33% extra damage and 43% extra health. Sif is unkillable and lacoste shoes uses powerful frost AOEs, making surviving much more difficult.