This Seeking Tech guide will explain how you can unlock the Jeanne d’Arc trophy called Might of the Therion.
You won’t be able to get this until you complete the 13th stage of the campaign
Afterwards, you have to use the three Therion members in your party (Le Hire, Rufus and Bartolomeo) to complete any stage.
The easiest level to do so is the Chapel in the Woods. You should be able to win the short fight in a few rounds.