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I'm sure a lot of you guys have already seen the video of the puzzle ring farming of the butcher. Well let me improve on it a bit for you
Puzzle Ring Goblin Farming 1.0.4 - YouTube for those who haven't yet.
Anyways. In the video they are using the Butcher. I have found that any of the act bosses will reset this. I'm not 100% on other bosses, but I believe it should work on any, I have only tried the act bosses.
So here's a few tips.
I personally use Belial in act 2, since at the start of the encounter it's only those serpent guards. Let them whack you, gg goblins.
When you are restacking your neph, if a puzzle goblin is popped out of you, you have to wait (edit)15 minutes for it to reset even if you do the boss encounter, so don't worry and wonder why its broken all of a sudden.
Act DOES change the drops. Say if you do it on Act 3, you are going to find a higher rate of 61-63, since it is Act 3.
I am going to test this on other act mini bosses to see if it works. I will get back to you all on it.
If you have any questions, I will try to answer them if I know.
EDIT: According to a blue post, the goblin is always level 61. It does not turn into a higher level goblin by being in a different act. However after 5 hours of testing, i've noticed a dramatic increase in iLvl on items dropped by being in act 3. Call it RNG, but i personally feel after hours of testing that the loot is better in higher acts.