The more challenging missions are the solo ones, so I'll share the secrets of some of my successes here.
Jean Grey: use telekenesis (hold B) to slam those cranky bipedal bombs into the cars, thus clearing them easily and quickly so there's more time to devote to the battle with the Grey Gargoyle.
Gambit: use 52 Pickup liberally to break open boxes, junk and hidden rooms hiding those pesky hard-to-find data disks.
Thing: stun everybody with Hand Clap. 'nuff said.
Green Goblin: Spinning Blades is insanely lethal. Repeatedly 1-shot kill respawing Negative Zone guards until very close to quota, then after setting a bomb in the middle of the irritating electrified room, run for cover and stand at the exit doorway, continuing to kill guards until the exit door opens. As long as Gobby remains in the doorway, he can't be electrocuted.
Venom: Same mission as Gobby, except without the bomb setting task. Venom is easier because the mission allows a full team complement. Just keep killing respawning guards until quota.
Songbird: The most challenging thing is to keep the scientists she is escorting alive through the whole mission. Cast Sonic-Scream Shield and refresh often to keep her healthy and enemies weak. When she gets to a door, use Crushing Discord BEFORE opening it to kill enemies on the other side. She can hit them through the door, but they can't hit her. While Crushing Discord is good to use on Light mutants, Shrieking Slice works better on Heavies and Moonstone (Boss). Alternate the attacks when facing Moonstone 'cos Light mutants are attcking the scientists at the same time.
So, getting the whole team was do-able after all. Now the next challenge is to attain the Gold bonuses: same missions, higher quota of points to aim for.