At the end of Guild Rank 6, you'll get a quest entitled "Looking for Peace" that tasks you defeating the Demon King in Atlio City Ruins. Once you get there, you'll have to face the boss who is quite tough (I was barely damaging him at level 82) but just keep emptying the break gauge and using awakening attacks to take him down.
After a few cutscenes, the trophy will pop. You'll also unlock this one along the way: