If you were in a 2- or 3-player game, and one foe has Martian Manhunter, which Super Hero would you choose to maximize your chance at beating him? If the set matters, then with Heroes Unite.
Fight Martian Manhunter?
It's hard to fight him from a sheer power standpoint, so you may want to pick a good 'early game' hero, and try to build a solid lead before he gets going.
Another good choice would be Killer Frost (Crisis3):
"Once during each of your turns, you may pay 1 VP or discard a card with cost 1 or greater. If you do, play a card in the Line-Up with cost 4 or less, and then destroy it."
Getting rid of the cheap heroes/villains should slow him down a bit.
Green Arrow (Crisis1) has a better chance of taking Villains before the Martian gets to them:
"Once during each of your turns, you may discard a Punch card. If you do, you pay 2 less to defeat Villains and Super-Villains this turn."
Doesn't help with the 'Hero' side of things though.