Think & count -> don't waste money and buy everything (esp. useless artifacts - You'll find better anyway) etc.
Save before every fight and replay it until minimal casulties.
This way I've managed to get this nice hero:
Just be careful and don't get into overkill fights when You're left with few creatures left.
btw, this screenshot is just before labyrinth to murok (I'm worried there aren't enough exp. levels
)