The biggest thing with Colosseum is making sure you have 3 unique teams each with a designated purpose or overall idea. There are 3 basic categories to go for that. First is Fantastic Damage, Second is Normal damage and last is some sort of defensive oriented comp. Such as a shield stall or a control heavy composition.
For Fantastic damage the typical form is some sort of Scare comp utilizing Sulley and Mike 9/10 times. You can use Hades and Jack Skellington if you have them strong enough but most of the time it will be Sulley, Mike and Zurg. Standard set up there would be as such: Sulley(Wo), Woody(Bu), Zurg(Re), Mike(Su), and Flynn(Qu).
Next for basic damage comps or normal you want characters that can study enemies to allow your auto attacks to always crit. Most of the time this will be Baymax but Barbossa cpuld once again work if you have him strong enough. A standard set for that would look like this: Baymax(Wa), Moana(Me), Judy(Fe), Mickey(Su), and Calhoun(Vi).
Lastly there are your stall and control comps either of which are solid for you. Both work to either stall out the last round of defense or prevent the enemy from doing much of anything. For control basic set up would be along these lines: Ralph(Ja), Wall-E(Da), Tia(Ya), Ursula(Ti), EVE(Ca). Then for shield comps you can look at something like: Bogo(Mr), Violet(Me), Quorra(Buzz), EVE(Wa), Calhoun(Vi).
Hope that gives you some direction. Obviously there are more comps that can work those are just basic archetypes to follow.