The strategy I used used stuff from the optional boss in the mirror which include a unique gem and additional party member.
I equipped the holy light and empress gems to Miyuki and replaced Saya with the new party member, though the latter is unnecessary.
The boss is weak to Electric so 2 or more non-Miyuki party members need to Orbal Spark.
Use Orbal Spark and the skill gained from the Empress gem on the boss. When the first statue is summoned, have Miyuki holy light it until is dead while the rest of the party either guards or puts pressure on the main boss. This way you should be able to stun the boss and kill the statue before the second one is summoned.
If you have Marvin, I recommend setting up Demonic Flames on the main boss also. My party was level 13 when I fought her with the new member being level 11. You may need to use some items (I had to use a rescue crystal on the new member since she's pretty frail and 2 grapes for general healing). Saya might be better than the newbie but I use her, she'll definitely be tankier.
Definitely a tough boss but this strategy made it relatively easy. I honest had more trouble with the fairy optional boss than her.