It is relatively simple to beat the Rock Bird using Justin's V-Slash to deal damage. If necessary, use Sue's Rah-Rah ability to heal. When your characters run out of SP, just attack with Justin and Sue and heal when your health is low. The fight can be made even easier by having Sue use critical attacks every round with Justin following up with a combo. If you keep your attacks focused and constant, then it's possible to end the battle without ever being attacked.