Visit the Old Sorceress' Mansion after clearing the Great Ancient Ruins. Once there, Sarah will mention that she needs to retrieve a notebook. If you have the hint or clue about the 24th treasure, it can be found here as well.

Head to the Hermitage and to main room to enter an optional battle against this secret boss:

Kaim and Sarah are fixed characters for this battle. Have some status protections skills, like Anti-Petrify, Anti-Darkness and Anti-Paralysis. This boss loves to use its own version of Darkness, which damages the whole party, making them lose MP as well as inflicting the Dark status ailment.

Persona has two sides. The normal face absorbs magic while the knight/helmet side absorbs physical attacks. Switch your attacks accordingly. It only has 12000+ HP but this battle can be very hard if you dont know the proper strategy. Casting all generate is good. If your casters have learned Double-Cast, then the better. Just make sure your frontlines have Casting Support skill to make the casters turns faster. Also, this boss has an instant death attack, fortunately, effective only to one target at a time. Just cast All-Cura if your characters have multiple status effects. Using All-Generate for all characters should be a good support for keeping their HP refreshed.

After the battle, you should also get Sarahs Notebook as a bonus for this battle as well as the corresponding achievement.