In order to unlock this achievement, you'll first need to have gone on at least four dates with Marion before starting the final battle at the end of Opus 8. (In order to go on a date, you must first invite her to a Night Event by talking to her with
in Marga, near the entrance to the castle. After raising her affection enough, she will occasionally invite you to walk around town with her 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 her Heart trait Fluttering Maiden
, as it is unlocked at the end of the fourth date.
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 Marion, 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 Yuma and Marion declare their love for each other and the achievement will unlock.