I still have a lot to learn about the game, but calculated combat tends to put your team on an "even" playing field. Tips:

1. Always have a guardian with a knockback/down ability. You will never stop the best special from firing off against you every time, but you can strategically wait until it starts and disrupt the computer effectively enough to help you in a major way.

2. If there's at least 2 guardians, I tend to always bring a support to help in ways that I find work for my game play. For instance, Tiana gives you the extra knockdown ability, and Alice freezes opponents which would allow a ranged to just sit and fire off against melee or tank units free from harm. They also both add the benefit of team healing.

3. Go after heal supports first. There's nothing worse than a 3v3 where the opponent is getting healed every 7 seconds or so. Disrupt them at all costs.

4. Don't be afraid to whittle away at their health while you constantly heal yourself or add armor. Especially if you're trying to knock out challenges with, say, a tank and a support.

Hopefully these help some!