In order to unlock this achievement, you need to have Kureha at Affinity Level 4. The fastest way to raise affinity is by partying with Kureha and going out to fight enemies. Once she is Level 4, head to the shop in the SBC Glocken and talk to Kureha. This will activate a cutscene. You need to do this quite a few times but you'll get the achievement in the cutscene before this one.