Wait, people dont know about this? This is all over every powerleveling community in D3 and ive seen countless videos on it. I swear this is a repost... will search around
http://www.ownedcore.com/forums/diab...ar-needed.html (Easy Gold and EXP - Torment 6 - No good gear needed!)
And despite what you said above i wouldnt consider it an exploit. The guy you have to defend doesn even have a hitbox for enemies (They can walk right through him) While enemies do make attack animations against him he was never meant to die or take any damage, meaning this was intended to be an autocomplete. Players just figured out that you can just leave it to do it on its own.