The Strongest Bond is obtained by defeating Kainé for the third time within three and a half minutes. The first thing you need to do is obtain all 30 weapons to trigger the ending. At the end of the third playthrough, you will be forced to fight Kainé. Begin with using a few magical attacks and she will fall, then a barrier will appear. There is no way to destroy the barrier, and Kainé will shoot magical orbs at you for 20 seconds. Evade the orbs until she stops before slashing her with your sword until you defeat her.