Damn, do I even want Frost Ace? Fusion is expensive, and I'm not sure he's much better than Black Frost (who, unfortunately, has shit MP).
He is a physical demon and physical skills are cheap. The only exception is Desperate Hit.
His lack of MP is more than balanced with his high HP.
You can give him "Mana Surge" if you want to increase his MP but it's not necessary.
e: also you want Frost Ace to make Demonee-Ho later. He has unique and one of the best skills in the game. But it's tricky to transfer and it's easier to just pass to your MC so he murders everything. It's a physical multi hit Almighty skill which doesn't need weaknesses to do massive damage and gain press turns.
Almighty skills usually don't gain press turns but Desperate Hit does. It's game breaking but you know you want it.