Ran on Dolphin Emulator 2412. Difficulty shown at the end.

The Power Plant is the best possible stage. The giant beam in the middle can be used to cheese the AI's lives if your lucky. It's also easy to pin the AI down by throwing objects at them. Spider-Man is more optimal as Venom & Hazmat have issues trying to super grab objects. Plus his air grab has him jump off his opponent. Getting him out of the beam as to not weaken him once the opponent dies.

Magneto is surprisingly very squishy despite his defense stats and he doesn't like to super grab objects thrown his way. Considering he lets his guard down when he flies at times, he can't counter anything thrown at him.