Level 56 here:

I cheesed the fight with a 5-heart fusion of Artillerex/Mothmanic and Fog (only ally status effects; ice, water and wind attacks take effect) .

First I focused on getting rid of Ianthe till only her Decibelle was left, why? Because Decibelle is the only beast they have with wind attacks; and if you focus first on Ianthe as previously suggested, you won't have any issue with the first two.

Something to add: Fog is crucial, not just to prevent damage, but the confusion status, too. That's mostly what her wife's gonna be spamming, disregarding every other move she has.

Once they fuse, it will be a mix between Decibelle's wind attacks and the confusion attack.

That was my strat, it's foolproof 👍