Go to Agart town then talk to the well in the middle of the town.

Bosses: Calcas and Brinas
Kill one Calca, then kill all Brina to trigger a second boss. If you kill all of them at the same time, you'll miss the second boss (again bestiary).

Boss: Calcabrina

Boss: Golbez
Cecil uses Libra to check his weakness, Rydia uses summon accordingly. Note: Golbez can change his weakness multiple times, so make Cecil uses Libra to check whenever he changed.