Well, I'm currently playing through the 3rd map with alternative, upgraded skeletons and Arantir, and I'm managing thanks to summons and berserkers. Plus, I summon the Avatar, and that's just insane! It deals an average of about ~700 damage. Of course, Darkness is the key!
How often does your Avatar get killed? I don't use it at high hero levels because of the chance of it dying, and I don't want to waste mana.