The main thing about this battle is that Mave changes her resistance every turn. One turn, she’ll resist magic; the next turn, she’ll resist physical. And she always changes her resistance after you’ve set your party’s actions for that turn (but it’s always one turn magic, one turn physical.)
However, only Mave’s resistance is changed – not her companion’s. Also, and I can’t be certain of this, but it seemed like Lightbringer and Lemming’s Pure Divine Blow (if I got the name right? it’s the one that does 999 damage guaranteed…), anyway it seemed like these skills hurt Mave regardless of her resistance – but I could be mistaken on that point.
So anyway, I spent a lot of turns having 3 characters use their strongest all attack skill regardless of magic or physical, while Milka cast All Cure practically every round. Sometimes Mave took damage; sometimes not – but the companions always did. I kept that up until the companions were dead.
After that, it’s just a matter of deciding who gets to attack each turn, and who will support. (Based on Mave’s resistance for that turn, of course.)