@Chase
I tried all three classes on impossible, that's why I moved to KB: AP, the russian version. Warrior seems to be the easiest and seems to suffer low or no casualties, because most of his mana is dedicated to buffing/resurrecting and with a proper spell and army setup, he rocks. He also gets big rage boosts while the armies exchange blows, which allows you to time back or use offensive skills from the spirits pretty much every turn.
I'd have to say that the paladin has the toughest going in my opinion, because he can neither multi-geyser or use timeback + underground blades like crazy. Some fights are especially tough on impossible and you need a pretty strong army setup and enough options to resurrect your troops. I can remember my problems with karador even now
As a mage, I left my mana just enough to 3-geyser and built-up some rage for underground blades/time back to a decent level (it's hard to max it without hindering your magic usage) and picked all the rage reduction costs. Still, the geysers were pretty much the thing I used throughout the game.
The thing is that I wasn't bent on doing all the games with no losses, but having the combat experience with both 3 classes, I can have a pretty good idea which would be the easiest... I think that would be the warrior.
@Elwin
The russians don't seem to be doing much on the dragon now. Some skills were fixed or balanced, but I don't seem to find much of a difference. Swooping dive or whatever it should be called in English (the mass-damaging dragon ability that pushes enemies back) still works like a charm. Same goes for the chargers and deadly blow (or whatever it is called, that damages and pushes enemies lvl 1-3 back 2 cells). Those are the abilities I use the most and they don't seem to be affected. Besides, I always have rage for them, since in AP, you have a skill that actually retains your rage in a manner that in can't drop below a certain percentage of your total rage. Even if you use up all your rage in combat, it automatically regenerates to that amount. Pretty neat.