Gunnr spawns adds that have aggro requirements. The adds are a misty warrior, a misty crusader and a misty brawler, they cannot be tanked by the class who's name they share. E.g. a misty warrior must not be tanked by a Guardian or a Berserker. If threat is pulled by any scout/mage/priest or improper fighter a fail script will occur killing all of a certain class depending on which add it is.
As with the easy mode Primus Pilus Gunnr, he will summon misty warriors, crusaders and bruisers. Once again they must be tanked by a fighter class of a different type (e.g. a misty crusader must not be tanked by a Paladin or Shadowknight). If the wrong kind of tank or any non-tank class gets aggro, they will kill all players of that archetype in the raid.
Gunnr also casts a curse that, if allowed to expire naturally, will wipe the raid. The curse expires quickly (7 second?). A raid with 7 or 8 healers should have little difficulty keeping ahead of the curses however.