I worried about the same thing when I played my impossible no-loss (or low loss :) ) games.

Good news is, I was able to go no-loss until baal (bleh, 52 casualties) using NO undead as a Warrior Class. I'm trying to avoid using the usual "boring" Black Knight combo to beat bosses. :)

Paladins, Elves, Hunters, Royal Griffins (for show), and Bowmen. Yes, bowmen. The little guys in blue suits.

The irony is, they weren't part of the 52 casualties. :) I only lost 2 royal griffins and 49 hunters. :(

However, I specifically went for a 100% critical hit build for all the archers, which required me to use Trigger and quite a few specific bow items. It also required the gift mod's black bandage.

You can actually buy a lot of human army guys right before the last battle. It almost seems like the game was supposedly designed to allow for a human army to win since they have a lot of physical resistances.

hmm bowmen/hunters sounds interesting.. probabaly will try it in my next run.
right now i am owning everything using. all dragon army supported by inquisitor , paladins , black knights. with Moro dark. focusing on high morale for all. with level 3 onslaught and morale 2/3 on dragons i just start hitting every thing on first move itself. use haste to move paladins/black nights forward. second round use special abilities of dragons and start hitting with stone skin paladins/black knights. almost all battles are over in 3/4 rounds.
vs demon/undead based army phantom my high morale inquisitor, and watch the fun.
game is almost over. :D