I searched but havent found anything about this so here goes.
If you find a group doing the arena quest in the ogre arena in north of nagrand, stand on one of the spikes that surrounds the place. When the quest mob spawns, use an attack that creates much aggro, once you got the mobs attention it will evade and piss the ppl off that cant finish the quest. Works best for warlocks using searing pain or other ranged DDs.
To finish the ppl off, get aggro and wait till the mob evades. If the others stop attacking it, use a spell to lower your aggro like invisibility for mages, FD as hunter and the mob goes back the the next thats in his aggro list and most of the times catches the group unaware killing them if they just stopped healing whoever tried to tank it. You may die aswell if you leave the spike once the group is dead.
I learned this trick when I was helping a group that was just doing the quests and evaded some of the mobs for a moment by standing on the spikes.