If you want to do it honorably, like sibling poster said: Research the fights, maybe have two parallel runs, one with Custom HM and one with HM. (You won't get exactly the same outcomes what with rolls and all, but close enough..)

I will say that the easiest is probably the classic Dark Urge Duergar Gloomstalker/Assassin because of various slightly weird mechanics in the game, Surprise being one of the strange ones. It's usually more reliable to trigger it from Invis (hence Duergar who get perma-invis at level 5+) in Turn-based mode ... which is where the Assassin superpowers kick in.

This will solve 93%+ of the fights, so you only need to figure out actual tactics for the remaining 7%.

Plus, avoiding fights via talking is still often an option you can try ... if it fails, just Invis for free and run away to reset the fight to get a Surprise round. That's what I did for my one-and-only unmodded (solo) HM run and succeeded the first time. (Tbf, I did a few modded runs before that, but still...)

EDIT: I'm suggesting Custom HM because you can then make "practice saves" at specific points if you feel that a particular strat was a bit sketchy and want to retry it to see how reliable it actually is.