Metaphor: ReFantazio

The first chance you’ll get to do this is inside the Northern Border Fort, after leaving the Royal Capital, Grand Trad. Unlike Persona games, you can actually engage in third-person combat as well as turn-based combat. The third-person combat involves hitting the enemy (in this case, an egg monster inside the fort) with the button, before a Squad! notification pops up (the button).

Then, once you’ve stunned it, and hit the button, you’ll engage in Squad Battle and stun the enemy, and thus unlock the ‘Stunning’ achievement.