Very fun, but I'm not interested in paying for it. Got all of the achievements.
Make sure to turn on 300% XP in accessibility to speed up grinding. The final level is easiest with two berserkers at boost 4, two rangers at boost 4, and two healers (if berserkers are taking damage). Recall and reposition berserkers when oz passes by. Spells are pretty much useless. Max out Frenzy and spend the rest on whatever you want.