In Sound Mind

In a few places in Allen's tape, essentially "level 2" of the game, his shade will attack you. He looks like a puddle of black ink, or maybe a puddle of darkness, among with a torso and head with glowing yellow eyes in the center of it that glides or "swims" along the ground and tries to suck you in, which then causes damage.

For this achievement you simply need to shoot the human shaped parts of the shade and he'll temporarily retreat and this will pop.

If you don't feel you can shoot it the first time you encounter it by the lighthouse, later in Allen's tape will be a wharf which you may have noticed on the area maps. In this are quite a few places where you can stand where the shade cannot reach you, but will still stay nearby for a few seconds and you should easily be able to shoot the humanoid form in the center.