I got this once I finished the game. Enemies in the game don't level up, so just go to one of the early stages of the game (e.g. altar), find an enemy and make a surprise attack (approach from the back, use Igniculus to dazzle the enemy if needed).
Once in the battle screen, select a character that has an "... all" attack (e.g. "slash all") and use it against your enemies (must face 3 enemies for the achievement to unlock). If you have a strong character and advanced oculi equipped, you won't need more than that first attack.