I can't tell you which medals to use since I don't own many of these but I can tell you what strategy I have used.
The key to this strategy is to defeat the cat that heals first (speed type). You can use the Treasure Trove (TT) to get a great boost for your power medals. You can use another keyblade too, if you think that TT isn't the best choice for you or if you don't have fitting medals for this setup.
- First of, you need to try to get your defense as high as possible, or you need to try to get the strength of your opponents as low as possible. If you have some medals that for example, inceraces your defense for 2 or more tiers, i would say two or three medals are enough.
- After getting sure, you won't die in one turn, use medals to get your strength higher or to lower the defense of the opponent. Again, about two medals should be enough.
--> Illustrated KH2 Kairi is pretty perfect for healing, dealing great damage (if you have her guilted - if not ignore this point), restoring gauges and of course for strengthen yourself and weaken your opponent.
- Your last keybladeslot can either be a medal to strengthen yourself a bit more, to deal damage or to be safe and weaken your opponents or get a bit more defense. This is your choice to make
- As for your pet, you should put in the strongest medal that you have. Since it dosn't matter what kind of type the medal is, you can put anything that makes great damage and maybe gives you a little extra boost. (I'm pretty sure your pet is at least rank 3)
If you have two medals with any kind of defense boost (the higher the better of course) it would be great. My Illustrated KH2 Kairi has defense boost IV but it did activate pretty rare (which gave me heart attacks sometimes). Only your damage dealer needs to have a attack boost.
I hope i was able to help you a litte
PS: You'll probably notice that my mother tongue isn't english so please tell me if something isn't clear