This can only be obtained after defeating the first Siren, collecting the Barnacles, and getting the Seer Dance. If you take damage for seemingly no reason, perform the Seer Dance with
to show any invisible enemies.
There is an invisible enemy shortly after reaching the Sea Lab.
Upon reaching the area, there is a save room, an empty room, then a room with a platform and two crystalline enemies. Perform the Seer Dance here to show an invisible enemy and unlock the achievement.