Killed an enemy with its own projectile.
Using telekinesis grab a thrown projectile and throw it back at the enemy. You can either do this against a Splicer or any other enemy which throw projectiles such as grenades or rockets.
One of the easier ways to earn this achievement is to kill a Rumbler with it's own rockets. You can stand in front of them and keep using telekinesis over-and-over until you kill the Big Daddy. Splicers randomly throw grenades, but the Rumbler will always fire rockets at you if you are at a distance.
This can also be done with Thuggish Splicers and Rocket Turrets.