Power Buildings

A super unit could then be any hero. Here's a super mage:D (from Haven)
But still, the Barbarian's resistance and ranged attack will give the Chaos or Haven mage a good beating.

No, they won't, maybe only against Chaos, but the Haven mage will cast spells on the Barbarian's army, not on the hero.
P.S. It will be fun if Berserk is cast on monsters standing next to the hero (especially if there are 30 or more of them:D) or, at worst, on Cyclops. Against Haven, the Barbarians have little chance, unless their army is ten times larger than Haven's.