Place the Frozen Heart AF to the space directly below the Bone Fortress to create Fieg Snowfields. Enter Fieg Snowfields.
Head to area 13 on the map. Next, listen to the conversion and talk to Mephianse at the campfire. Finally, proceed to area 18 to fight the boss.
Upon Completion: Return home, talk to Lil' Cactus, go downstairs (watch him write the Event on the leaf), and go back upstairs to save your game before continuing.