If anyone is having any issues with the hidden boss (Youma of Sloth: Roselia and her asshole Apostle), here was my setup:
Level 34 Otome (Father's Staff, Exorcist's Gloves, Electrical Headband, Full Exorcist Garb, Imagination Ring)
Level 34 Mei (Youma's Hatchet, Refined Gloves, Platinum Hair Clip, Succubus Leotard, Talisman of 8 Million)
Level 30 Hibiki (Priest's Mace, Exorcist Gloves, Exorcist Ribbon, Succubus Leotard, Beads of Protection)
Level 30 Koyuki (Father's Staff, Refined Gloves, Bio-Helmet, Gundarium Armor, Lightning Badge)
This is easily the hardest boss in the game, high HP and you're fighting two at once.
-If you have multiple Lightning Badges or Fire Resist Accessories, use them or get more Talisman of 8 Million. Lightning Badges because on my third try, I had gotten Roselia down to a sliver of health when she whipped out a multi-Lightning storm and Fire-storm spam that paralyzed my entire party and killed me on the next turn while they were recovering.
-Make sure to grab the 'Sealing Holy Water' from the shrine. I know it only has a quantity of 1 next to it, but you can use it an infinite amount of times.
-Apostle is a lesser worry, but he can do a rather nasty group attack and will occasionally buff his and Roselia's attack up in one spell (shame that Koyuki never gets access to single-spell multi-party buffs compared to every other frickin' enemy in the game.) Focus on taking this guy out first, don't bother with the holy water.
-Mei was my fastest in the party and highest damage dealer next to Hibiki, but that agility was more crucial as both Roselia and her Apostle can attack multiple times in one turn and have a fairly high agility (that I think increases when their health goes down past a certain level). I had her on emergency healing item duty (Healing Cross and First-Class Sushi)
-I always made sure to have two of my party members doing defence, even when I brought the apostle down, as Roselia has a tendency to spam multiple high-level spells that can potentially one-shot the entire party.
-Only used single high damaging attacks (like Iroha for Otome or Chrysanthemum Star for Mei) and then had them on defence (or healing if needed) to wait out their cooldowns. Hibiki has a really powerful single attack called 'Execution' with a decent crit rate that I would throw in every now and then Koyuki was on permanent mass group heal.
It's a long slog and probably the hardest fight I've had in any HRPG that I can remember (closest I can think of are the Scale Garden Games optional bosses) but it is definitely doable and proves that your favorite sluts-er strong PCs-can take any sort of pounding and come out on top!
Hope this helps!