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.