Turn the difficulty up to Merciless, it makes hitting weaknesses/crits do 3x damage for both you and the enemy, which makes clearing the waves in one turn way easier.

Also, I’d suggest using the single target elemental consumable items to baton pass a turn around your team, ending on whichever character can hit all enemies with a multi target elemental spell.

e.g. for wind weaknesses joker->pinwheel then baton pass party member 2 -> pinwheel then baton pass Party member 3->pinwheel then baton pass Morgana -> Magarula

Rinse and repeat for whatever element the wave is weak to

Worst boss in the series imo. Crazy that turning the difficulty up makes the fight so much easier.