I hit nightmare with my friends and at first didn't really notice the difference until I encountered the first "Arcane" boss. I sc!!*#d out of that fight at 25%. Started to think that I might want to slow down and do a few full clears before we get to the Butcher.
I play as a Monk. I typically play with some buddies. Usually a Barbarian, Witch Doctor, Demon Hunter.
So pacing questions.
1. How much health do you recommend before attempting the Butcher on Nightmare?
2. Should I be pacing myself? Is there a good place to "montage" in act 1?