After defeating the Grand Spire and enlisting the help of The Hero of Skill, the final but albeit brief showdown with Lucien takes place in the Spire. Here you will be offered 3 choices, each with an achievement. For this achievement choose the "Sacrifice" card.
Note: This and the following two achievements can be earned if you save before this final confrontation, choose one, then quit out either by powering down (or go to the dashboard, hit and sign out). Then reload your save and choose the other. Repeat one last time to have all three.