You can unlock this achievement as soon as you recruit Ignus to your party. Pull LT and select "Talk", then click on Ignus to speak with him.
Ask him why he's following you and taking orders from you. You'll discover that Ignus' original master in the Arts was you in a previous incarnation. Pursue this line of questioning and keep pressing him about this until you get a dialogue option to recall a memory. Click through all the memory dialogues, then complete the conversation with Ignus.
Your achievement will unlock as soon as you exit the dialogue.