[seriously, SPOILERS. Go away if you haven't beaten the game]
Anyway, I've heard from a lot of places that there's exactly four fights in the game that you have to actually finish by killing someone - everything else is either able to be skipped by not being part of the mandatory plot (sidequests), or you can talk your way out of it, stealth past it, or just run away from it. The problem is that I can only think of one fight you can't skip: Trias, in Carceri.
What other fights in the main plot are completely impossible to finish except by killing someone? --- Can you see a vampire through a one-way mirror?
Ignus or Vhailor, depending on your alignment. Vhailor appears if you're evil. Even if they are still trapped or were dead when you last saw them. TTO frees them, revives them, summons them, whatever that cut scene can be interpreted as. --- No locked doors, no windows barred. No more things to make my brain seem SKARD.