You must complete "The Final Batlle", which is unlocked shortly after you start looking for the 3 AI.
The mission takes place in the Central Plaza, where you and the Corps must defeat the Rogue Knight. I was level 41 and using the Nova Sword. Stats don't really matter, as if you get knocked out, you wake up, still in the battle, with some health (I even uses the item dupe glitch to get 1000 defense, and still got KO'ed, so don't worry about healing items).
The first stage consists of fighting the Knight atop the AI. Bring their health down to 70% and get a short cutscene.
Now you'll have to target his left leg, this will drain his health quickly.
The last section is against the Knight himself, pretty similar to the fight in the ruins, just wail on him until he has 20% health. After which a cutscene will play, and you'll earn the achievement.