Blue Dragon

Pull the right trigger to open the encounter circle with two or more mobs in it and scroll down on the right tab to fight all enemies. Does not need to be a monster fight to get the achievement.

For anyone wondering, the earliest possible time you can get this is at the 'Mechat Crash Site', just after you escape the Mecha Robos. You'll be shown a few pop-ups detailing how to initiate a multi-monster encounter. Despite the name of the achievement, you'll get this immediately after triggering the battle regardless of the outcome.