Those upgrade fights are tough - but I found that the Evil Shoal spell is especially good, and better when it's upgraded. You have to fiddle around with the target square to try and get as much killed as possible. You've got to take out at least one tower on your first turn...
I make sure I have lots of rage before attempting these fights. There is a potion you drink that converts mana into rage. This is very good for a warrior character, because you still get kick ass rage spells while ignoring the actual spellbook.
At the end of the demo I had:
5 Knights
5 Archmages
5 Horsemen (this guys kick ass)
30 Swordsmen
13 Ancient Bears (not my favourite, but I ran out of other units)
With level 6 rage spells, I was able to take down a few upgrade fights.
