Yeah, for now, sorry, I can be wrong:
finally with lasts balance, some bosses are less (no longer?) boring stoppers preventing from continuing the game.

Weird!!! i have np at all with the bosses and i played it a long time on steam :P.....
maybe learn where the spawpoints are what he uses and how to zigzag evade some shit....

I have a peculiar problem with Kupacabra (or however this vampiric leafmane nemesis was called).
Problem being it's unable to strike me down - my resists, 100% armor absorbtion, shitton of armor, overguard, hp regen etc etc see to that with ease. And likewise, I am unable to outhit its health regen and/or vampirism. After half an hour of pointless struggle, I just decided to disengage.
It was chasing me through half the Ugdenbog, finally resigned and decied to camp for me right against the "gates" of the coven. Empyrion bless the being that made the riftgates and have placed them throughout the world.

All in all, I have slain 4 nemesis monsters already, and I'm thoroughly disappointed - slaying them is just not worth the time and effort invested. From the first I slew (the electrified zombie thing that is hanging around Necropolis), all excited and stuff, I got one meh-ish MI, two random blue items and a few greens. Way to kill the mood.

If you have the possibility, just avoid, or if not possible just run (which still might be problematic, since some of them do have charge/dash/shadowstep/teleport and/or shitton of stuns to hand away).

Yes, I'm still near the beginning, more or less, lvl 15, and every boss I've encountered I have whipped with no trouble at all. I've only been killed once--when I took my eyes off the screen during a boss fight and forgot to hit my health potion button...;) Came back and clobbered him.