How to kill Baal; mage on impossible

[fable]

I also couldn't kill it fast, my solution was to scale up with the increasing stacks of demons. With Frenzy 3 you get higher and higher attack. Only bring units you can keep at full stack: paladins, black knights, ghosts (!), vampires. A neat trick against big stack of archdemons, when they halve your stacks, is pain mirror, then turn back time.