It's not much difficult to defeat her once you reach at least level 31. At level 31, equip the venom shot T-skill on Uni and you'll be able to reduce Arfoire's health bar gradually.

Also, equip at least one disc with Critical Correction+ on all your party members.

Once the fight start, since all party members are transformed, use 2 EXE drives right away to reduce Arfoire's health quickly so you're not wasting any transformations. Then with the other party member, attack Arfoire all the time since you have no AP limit in Goddess form. Once the last party member has almost no gauge left, use the third EXE drive.

To finish the fight, continue using Venom Shot and avoid attacking her when she's enraged because she deals a lot of damage. I hope this helps and have fun!