Well the way it worked in FFXI was that the "warrior" type char had an ability called provoke which caused massive hate or aggro. It was refreshable every 30 seconds. so you used that and he kept all the hate on himself. Also the amount of damage you inflicted effected your hate levels along with cures. whoever had the highest hate against them was attacked by the monster. It would be very interesting to see this implemented in XII because it would add that extra layer of strategy and dificulty to the game.