If your tank is getting to 5% from pulling the boss something is REALLY REALLY WRONG. Dungeon bosses flatly dont hit that hard. The only way this happens is your tank is woefully,absurdly, comically undergeared for the leveling dungeon. The guy using a lv 14 weapon, a lv 20 int ring and some 'disciple of war' lv 17 armor in a lv 30 dungeon kind of bad. Tank busters dont bring a tank to 5% with zero CDs used in dungeons let alone the 1st auto attack.
These 1 hit 5% tank stories are pure hyperbole. A once in a lifetime experience. Hardly an argument to support the claim.
