You cannot proceed any further past B1F of the Golden Lair until you have initiated this mission. Once you have done so, Kibagami challenges your party to a battle to prove your strength. Defeat him, and not only will he allow you passage further into the Golden Lair, but you also get the Twinfang Scroll, allowing you to set subclasses to your party members. Then, proceed past the uncharted exit in the Sentinel Village to the other side of B2F of the Golden Lair. There, your main goal is to maneuver to the large scale sitting in the center of the map.