In order to unlock this achievement, you'll first need to have gone on at least four dates with Lestin before starting the final battle at the end of Opus 8. (In order to go on a date, you must first invite him to a Night Event by talking to him with
in Marga, east of the entrance to the castle. After raising his affection enough, he will occasionally invite you to walk around town with him at the end of Night Events.)
If you are unsure whether or not you've gone on four dates, check to see whether or not you've got his House trait Elise's Disciple
, as it is unlocked at the end of the fourth date. Note: According to a Japanese guide for the original game (Shining Resonance), Elise's Disciple may not unlock until the fifth date for some people. In that case, as long as you have been on four dates with Lestin the achievement should still unlock, as the game counts the actual number of dates, not the corresponding traits.
After triggering the event in the Lost Garden in Clavalle Crags, Yuma will think to himself that there's a chance he won't survive the final battle, and that he should talk to the one he cares for most so that they know his feelings. At this point, all of your party members will be standing by with hearts over their heads (assuming you've gone on at least four dates with them). Choose Lestin, and then proceed with the final battle. (At some point after defeating Georg, the game will ask you if you want to save; feel free to do so, as loading the save will place you at the entrance to Lost Garden just before triggering the final battle event allowing you to choose other characters to see their endings as well.) After the end credits, you'll get an extra cutscene where Lestin invites Yuma to return to Wellant with him and the achievement will unlock.