Let's talk some PvP Strategy. What I'm playing at the moment, why it works, how you'd beat it.
My agent carries: Heavy Ion Blaster, Quantum Jumper, Coulson's Revenge, and The Enchanted Crowbar The idea is to make everything work together.
Colossus is the tank. It helps that he can hit hard, but his main role is to buy the other guys time to get their combos off. He can add two debuffs (Exposed, Slowed) that both make my guys hit better and can be exploited by Mockingbird. He can also exploit a debuff (Exposed) that he or the agent can lay with his Decimate move. But mainly, he keeps Mockingbird alive.
The Agent does several things. Combat acceleration through the extra turns of being an agent and the quantum jumper. DoT via radiation from the HIB (sadly, this is not exploitable by Mockingbird), applying debuffs with the crowbar, getting an Avenger Bonus via the Coulson (oh, and killing someone dead with the Coulson).
Mockingbird is the heavy hitter. Mockingblow will kill just about anyone with 3 non-bleed debuffs. She does less extra damage on bleeds (because they can stack) so, given the choice of Exposed or Bleeding x2, I'll take exposed. She also does combo setup, which works with both of Colossus's 1-on-1 attack moves.
You will notice: No ability to heal. No shielding. There is no need. I am killing most teams by the end of turn 2. I don't get to turn 4 often at all. Colossus can die because he's really the third threat. MB is the main threat in this construction, and it would be a good idea to kill her first if you had someone with stealth attacks.
I can even trust the AI to play this acceptably. Colossus's passive always works and he never breaks it by using his Steel Curtain move.
What bugs me: Stuff most people don't play. Black Widow and Black Panther can break this strategy by debuffing Colossus, which increases his damage taken AND stops his protection, letting people hunt MB before she gets setup. In theory, Kitty Pryde could present a challenge, as she could start phased and countering, attack MB with Lockheed in the first round, be unhittable for MB, then do a large hit via Stealth Attack on turn 2, possibly killing MB, negating her extra turn from class switching. Avengers teams that get 5-6 extra hits can be troublesome, but aren't worth mentioning. Blaster Thor is generally pretty dead via Coulson, Crowbar - MB in the first round.