I heard that even if you're maxed level, you'll still have a hard time dealing with her.
Came home and just cleared it.
I did ng+ since we only need to access the estate to start this boss.
My ng+ has better stats and all the 1st playthrough equipment and that was enough, although I spent Spirit Soul Stone (full hp party revive) and a lot of enhanced revivers.
Lv84 Polca, Lv66 Naje, Lv76 Lily (a lot of books on her) and lv76 Ange.
Equipment:
Polca: Goddess whip, Sheld Barrier, Old Crown, Cloak of Everlasting Darkness (final dungeon so you need ng+ for this), Suicide Dagger (penultimate dungeon boss, so you need ng+ for this).
Naje: Silver Lace Spear, Kraken Bone, Ape Crown, Mana Armor, Winged Necklace (Forgot when I got it) and Older Brother's Pendant
Lily: Crystallized Abyss Sword, Dragon Buckler (bad idea, I forgot to give her another shield), Mushroom Head, Princess Dress and 2 Imperial Cape (ng+ for this since penultimate dungeon gives you enough imperial capes).
Ange: White Silver Lace Spear, Gold Buckler, Mind Cap, Cloak of Radiant Lignt (Penultimate dungeon, so you need ng+ for this), Mother's Pendant and Battle Princess Necklace.
Turn 1 is pure RNG: Apostle Polca. Everyone does their thing and Ange uses her Deflect. Nobody died in my run.
Turn 2 onwards: Naje spams Full Aura, Polca spams Mei Dew, Lily uses her poison on King and Ange goes from Wind God Shield and Super Deflect. Spam a lot of revival items when you need it. Sometimes, you have to pull someone from the reserve row to help with revivals. When things are dangerous, use normal Polca and Naje ultimate skills. Polca should go transform back and forth depending on when she needs to use Full Aura or not. Transforming doesn't use a turn after all.
Once King dies, focus on the right crystal with the same strategy. The cyrstal will do a lot of Aoe Phys attack now and it has a lot of accuracy so Ange will die a lot. I actually lost twice here.
Once the right crystal dies, you have practically won. Swap Naje or Ange with Kalinka to make things quicker. Naje gives better damage with the crit buff while Ange makes things safer. The left crystal can't be poisoned, but it can be silenced and its phys attack is super low so either Naje or Ange spams silence, Polca heals or buff depending on the situation while Lily and Kalinka destroys it. Kalinka buff stacking is a thing of beauty.
Only 1 boss more, the normal final boss but buffed from ng+ if I am not wrong, but I will do it tomorrow. I don't think I will be able to beat it without the ng+ equipment anyways so I will have to use this save instead of the 1st playthrough save.