1) Minute Man does truckloads of damage with Smash, and will often deflect bullets even without his active defence, but he can get worn down by multiple hits from range. If you're facing a bunch of foes, fall back into an alley and wait for them to come around a corner, or pick up an object and crush them from a distance. Don't waste time with the Minute Missile against multiple weaker foes- save it for enemies like Pinstripe who are mostly immune to Crushing damage. You can also always jump on top of a building to escape bad situations.
2) Mentor is slow, vulnerable and has one of the slowest close combat attacks in the game. Luckily, Instinct Dominance doesn't need line-of-sight and has a huge range. Hide him behind something and target groups to get them fighting each-other, sending in Minute Man to mop up when they're distracted. Don't bring Mentor into fights against robots unless you've gained his Electron Beam.
3) Although it's tempting to fly into battle, El Diablo is a big target in the sky, and takes longer to get his power back. Fly him where you need him, land, then take off to close with his next target. Also his Tongues of Flame is much more accurate if you move a little closer to your target. Although his fire attacks are great against Nuclear Winter's cold minions, he is far more vulnerable to cold damage, so let Minute Man take point and snipe with El Diablo. Inferno is a great power that can wipe out whole groups of weaker foes, but try to use it in the air. A downwards arcing Inferno is easier to aim and won't prematurely detonate near you.
4) Man-Bot is tough but so, so slow. SO SLOW. He is only effective up close too (Focus is not that accurate) so you have to charge him in. I usually concentrate on getting him Flying just so he can keep up with the rest of the team. Watch out for Pan's electrical minions and just hammer away at everything else.
5) Alchemiss is quite powerful, more because of Repulsion and Alteration than her inaccurate Arcane Bolt and wimpy Smite the Wicked. She is great at flinging away or messing up targets with these powers, allowing your hitters to smash them. Keep her back from the front line, Repulse anyone who closes with her and use Alteration on the main threat. The results are amusing.
6) The Ant is a powerhouse. He is great at a lot of things. Up close he can out-fight almost anyone and Shove tougher targets. Team him with Bullet and Shove a target, then run in Bullet and hit them three times before they can even stand up. Acid bomb is great on groups- aim halfway between you and the target to get them with the bounce. Ultrasonic Squeal is also great for groups if they get too close.
7) Liberty Lad and Sea Urchin are fun, but not that tough. LL's grenades make him a decent range support character, but he doesn't have the toughness for long-term close combat. Sea Urchin's weird ranged attacks are unpredictable-you'll hit allies or civilians half the time with her Bubble Swarm, and Sonic Shriek is pretty weak.
8) Bullet is amazing. Just keeping your distance and making a whirlwind, then running in to punch your stunned targets can beat most foes. His Fist Fury and Fist Fusillade often stun targets. Prioritise targets who have good ranged attacks.
9) Man O' War is another powerhouse. He is very good at crushing and electrical attacks, making him a great pick for most match-ups and necessary against Mr. Mechanical.
10) Law & Order are still fun characters, they're slow and I feel slightly less useful than the rest. Law can sort of heal your people but is a mediocre melee fighter while Order is a slow-moving expert on dealing Crushing damage and not much else. It's neat that you get both by taking one, but even so, they're not so great.
11) Microwave can deal a lot of damage but again, so slow! Radiation is his main attack and it's very dangerous to certain enemies, and he is very resistant to most damage except electricity.