Equip Frog with Masamune and the "Hero Medal" (so Frog critical hit rate will raise), and Crono and Lucca must heal the others (you should take Marle, cuz Aura Whirl is a great help in this fight).

Keep this strategy until Magus "risks casting a spell" (or something like that), he will stop using that Barrier Change, and you will be free to use any spells you want.

Until he stops using Barrier change keep this strategy:
Frog - Attack
Crono - Attack, Mid-Tonic (Aura Whirl if you have Marle)
Lucca - Mid-Tonic