Are you playing it on easy? I make 2 swordsmen and 2 archers to start. Then focus down the mercenary building. If you have enough gold slow down the orcs with spikes. Start making the elves to replace your archers and send a few swords to protect the elves. I put all my shields into either the unit buffs or the money buffs
The key is using Blizzard. Try and build up a mass of units, and when the boss is about to start fighting your troops, stun him with blizzard. You should be able to kill the boss with minimal losses that way.
so far i didn't have problems with that level, just massing my lvl 3 archers and 1 or 2 spells and he's dead. make sure your archers are spread out so his splash attack doesn't kill all together. i mainly used archers so far investing my points in food gold xp and unit upgrades. so max xp for archers. sadly i won hard mode in 1:30 without any minotaur spawning so i have to try again.
can you win without upgrades and levels? just finished the human campaign, though i still need to finish the last mission on hard, but for that i'd have to do every other mission on easy to collect shields.