Each medal has a type that is stronger against another specific type. Power is red and is strong against speed. Speed is green and is strong against magic. Magic is blue and is strong against power. Using the stronger type against a particular enemy to get a damage boost. Bring medals built around the type of heartless you will be fighting. Medals can be leveled by sacrificing other medals. Sacrificing the same type of medals will also result in earn extra experience for your base medal. If you can get Huey, Dewey, and Louie, level your favorite medals with them. They are worth a large amount of experience, and they are not actually usable in battle. Huey is power, Louie is speed, and Dewey is magic.