There's a lot of noise about doing Beliel powerleveling (Act 2 end boss). Since I don't know if it's intended, I'll just link to the thread on the official forums about it: http://us.battle.net/d3/en/forum/topic/5626190683#3

Stuff gets nerfed all the time, so no idea if this will last. But it seems like it's REALLY fast, and I'm sure you could do it as a group.