This Seeking Tech guide will explain how you can unlock the Lethal Blow trophy in Jeanne d’Arc.
For this trophy to pop, you will need to defeat one enemy that has full health with one attack. You will most likely get this during the second stage.
When Jeanne gets hit here, a story sequence will play out on your next turn.
As a result of the transformation, her attack power will increase. She will also be able to use the Godspeed ability that gives her another turn when she defeats an enemy.
After her transformation, attack any full-health Orcs with Jeanne. Make sure you target their backs or sides to maximize the damage input.
If you didn’t get this trophy in the second stage, then you can go to the Chapel in the Woods and then use your transformation there. If your level is high enough, then you may not need to transform in order to defeat full-health enemies in one blow.