Just finished Act I on nightmare solo. OMG IT WAS HARD. Like, some of those blue outline packs of enemies will seriously fuck you up with status effects and damage so quick you die in 3 seconds. The single enemy gold elites are usually easier because it's just one of them.
The Butcher was pretty tough with my guy. I had to just manage my equip/skills so my DEF was so high and I could keep stunning between slashes with seismic slam thing + chance of stun & ground stomp stun, while absorbing HP per hit to regen since there was no way for me to be fast enough to dodge his attacks.
My problem with my character (barbarian lvl.36) on Nightmare, and why it's going to be tough to make it through the other acts solo is now that I have a ton of HP (~4000) and the enemies hit REALLY HARD (so I'll take 1000-2000 damage in a few seconds), leeching 100-200hp a hit (a hit every 1.5 seconds) is just not enough. When I had 2000HP, leeching 100-200HP + 20-30HP per second regen was enough to keep it going; also enemies on normal didn't HIT FREAKING HARD. But now the orb slooowly regens while every hit I take from a big enemy knocks it down dramatically. What I need is to get some loot that really ups my HP leeching so instead of being at 1% of the damage I give, it's at like 10-20%.
But I haven't gotten anything decent loot-wise since I started Nightmare; which is making it tough. I dunno if it's just the Act I enemies, but hardly any rares, just blue stuff. When I was in Act 3/4 with Himuro/Viz/Great Rumbler on Normal we were getting a ton of rares left and right. Maybe it's because you blow through stuff faster in multi and kill more things that you tend to get better loot.
Will try some new strategies tomorrow, or just pair up with other players.