Clear the first room before engaging. Any Ethereals left alive will aggro along with Pandemonius. The person holding aggro may use any combination of anchoring and kiting. When anchoring, tank in the center of the room. Put your back to the big hunk of stone in the middle, as the boss does a knockback. It is also advisable to have your healer secure himself behind a wall or object so he doesn't get knocked so far back that he can't reach the tank in time to heal.