I'm playing with a mage in normal difficulty, and it is really easy.
If you have a little luck, and get good spells early, you can kill much tougher enemies than your level or army predicts.
At the first levels, fireball is awesome.
After you find demon gate,you can win entire combats without an army! You just need 5 1 pcs. units (like thorns) to survive the first round and after that the demons take care of the enemy. The losses doesn't matter, and you can use magic spring to replenish your mana and summon an other stack if one or two isn't enough.
Firestorm is an another kickass spell. At 20. lvl there is no army what can survive 4 from those in 2 rounds (3-4k damage/pcs)
Now I'm at level 25 with an army of demons, red dragons, archdevils and other high HP units. If some of them actually dies, it is easy to get them back: Hypnotise an enemy unit, and then sacrifice them... Double win!