As I spent way too much time doing this level, let me give a few hints for those who have not done it yet:
Stage 1 - catapults & co.
- head directly for the catapults (they will take out their own team)
- catapults can destroy themselves (sacrifice a soldier to each)
Stage 2 - beam towers
- tank in the middle (shield with hero and heal him)
- archers attack towers (make sure the beams are focused on the hero)
Stage 3 - warlocks
- hack and slash (but not the door)
Stage 4 - chieftain
- focus fire on the chieftain immediately