Try your new strategy in the monster arena first, please.

You'll find that it's much more difficult to get anywhere with the magus sisters against say, Earth Eater? than a well made team. What with Cindy being so dumb and not using auto-life enough:[ and sandy being dumb and not casting enough Protect:[ and Mindy being dumb and forgetting to use Passado.

Delta attack is hardly worthwhile on those bosses, really.