Added after 12 minutes
Oh, yes, one more tip: you need to fight the boss exclusively with ranged damage units (archer masters + unicorn horn; druids + unicorn horn). The rest of the squad should consist of units with 1 hit point. If you've leveled up a Battle Druid (I don't know if it's possible), then that's even better. The fight involves casting a mass haste spell with the hero; then, send in a single unit, say, a Dryad, who will endlessly protect the archers and, if possible, the druids. (if the battlefield allows). The boss is defeated quite well; he didn't even get to any of my normal units.