1. Save immediately after defeating this boss.
2. Do not approach the statue in front of the party.
3. Take everything you want from Davy, Oscar, Sienna, and Rodrigo because they will be leaving the party and those Relics increasing Skill Use counts are very valuable. They also may have some great armor like plate mail if you bought it before.
And don't forget any good accessories, because the 2 new members will need everything you can acquire.
4. Now you can approach the statue and advance the game.