I think they could try implementing something like in Diablo 3, whenever another player joins the game, the mobs gain hp, defense and attack buffs. So say, the more players on a mob's hatelist, the more powerful it becomes. The more people try to zerg it, the more powerful it becomes.

They would have to cap the attack buffs to prevent even the auto-attacks from becoming one shot, but the HP and defense should just go up limitlessly.