Shurdus - sure some chars are easier and some aren't to play against AI.
Reguluses problem is... he dies a lot. Reg don't have any speed skill, dont have escape skill, so its harder to fight against cheating AI (with much better items). But still it's possible to win, if you play game right.
The question here was: is Nightmare AI beatable ? For me, clearly, answer is YES IT IS ! Maybe AI is cheating, but AI don't think.
UB and Erebus were just WOW in this game. Rook was good too, but only late game, when he got 1700 Hammer Slam. He never misses HS, even if you run 3 meters away (is that a bug? looks like, but maybe it's better this way).
BTW: I never play minion Erebus Don't like to control minions (and this game is about fun, so I don't play things I don't enjoy). So during earlier Nightmare game, I was classical Erebus.