Located in the Old Sorceress' Mansion basement where you fought Sarah before. Before you can access this boss you have to have the 4 poems found throughout the mansion. As you approach it, you can hear it moaning in agony.
This is a difficult boss if you are prone to just attacking without a second thought as it has an ability, Reverse, that allows it to either absorb all physical attacks or all magic attacks (including Force, Gamble and elemental bombs) as healing. Persona is ironically weak to fire, despite it using Sarah's Notebook (an accessory that gives the Absorb Fire skill) as it's body's base.
Persona is very quick, and can reverse multiple times before your party's turn is over. A successful strategy is to use only two characters, one magical and one melee, each with very fast attack speed. Since Kaim is required for the battle, he is a wise choice for melee. Use other characters to heal, and be wary of Death and Paralyze. There isn't much need for physical defense.
This event triggers after upgrading the Nautilus on Disk 4. Waiting for the White Boa is not required, so you can return to shore in the Nautilus by turning around and traveling the long way around the continent to a landing point after the tornado cutscene at sea.
Notes: Both Kaim and Sarah must be in party; when facing the party. It will absorb magic, and when turned around will absorb physical attacks. It will use Death to kill a single party member, Shinus to hit and blind the party, All-Stone to add Signs of Petrification to the party, and its Special attack Darkness will severely damage each party member, as well as drain away MP and apply status ailments to everyone.
Note: You need Poem Fragments A,B,C and D for this to work.You also have to clear the ancient ruins area before taking on Persona.