This is not an amazing exploit, but i dont think blizzard meant for you to be able to do this so here it is: (Ill upload correct screenshots later when im at home)
In the first parts of Act 2 just after you leave the main city place, you can kite any elite/rares from the big open desert area back to the city, all the way to the back through a door way and one of the NPC's start attacking it in the doorway, the NPC can not be killed and the mobs just get stuck in the doorway attacking the NPC where you can stay at a range damaging it down from a range without getting hit.
I realise this is useless without screenshots but i will upload later when i am home. If someone understands what i am talking about then hopefully you can explain with a little more detail until later on when i can provide screenshots.