an idea i've had today:
Angel of Retribution
But John, you are the demons.
and then John was a Cyberdemon.
Pros: You are the Cyberdemon. You sit in the arena waiting for marines to come so you can shoot rockets at them. Different tactics could be developed for different types of marines.
Cons: Have you ever fought CD in AoMr? Getting to him is probably harder than killing the poor guy...
Marines could be taken from your Mortem or generated based on a decent build for certain challenges (AoB, AoMr, AoSh...). Nukes would be placed on the ground once triggered, and if you get to them fast enough, you can shoot them out.
new tactics could be developed, for example, the problem with AoMr is that they can shoot and reload faster than you can even move, let alone fire/reload. What you'd have to do would go behind a corner, and employ a "prepared" tactic, that allows you to fire at the marine as soon as he comes into view, sort of a reaction fire if anyone's ever played XCom.