I believe this is Garuda, yes? If so, then judging by that screencap you're doing it all wrong. The easiest way to beat him is to have 4 dragoons. Equip them with Wind Spears if possible. You should have found some dragoon equipment -- both armor and weapons -- before entering the tower where he's located. You probably don't have enough money to deck everybody out, but you probably don't need it, either. Just get 4 dragoons, equip them with Wind Spears, then spend some time building their skill levels. You don't need to do much, so do whatever you think is best. When you've done that, get to the boss fight, and just have everybody jump. Garuda is weak against wind, so with the Wind Spears everyone will do a ton of damage.