Randolph
Warlord
All of the stack damage spells (ring of flames is the prime offender IMO, but others including pyre zombies are also problems) seem overpowered on higher difficulty levels (emperor+) right now. The AI will have stacks of hundreds of units, such that even catapults, fireballs, and other collateral damage units still only affect a small proportion, while damage spells like ring of flames can do the equivalent of killing dozens of units per cast. A single ritualist can be worth 50 champions when dealing with big AI stacks. Unless the AI can really learn to deal with whole-stack damage I rather doubt they can ever be balanced.