After I bought all of the new P5 costumes (only the ones that costs less), and played with them a little, I think I found some good combinations.
1. P5 Pheonix and P5 Cyclops- if you get the pheonix buff of both of them in the same turn (it can happen, happened to me), you get a gauranteed hit and critical. If you combine that with Pheonix Flare + Arc Reactor Charge, You can destroy everyone!
I need more time to find more combinations, but I think I should farm CPs to buy the alternate class P5 costumes.