To defeat the first Gaia you will need free experience. When Gadwin and Sue leave your party they leave experence in books. Use these books on Justin and Feena. Concentrate on attacking Gaia's body, not the arms. Once the body is gone the Gaia is destroyed. Use Midair Cut, Burnflare, Milda Kick, and Rapp's missle or discutter will finsh the Gaia quickly. Just to be on the safe side heal as much as possible.