The achievement requires you to actually kill an enemy and not just deal damage (the description is misleading).
You can do this right in the tutorial when you first encounter the wretches. Have Gabe position on the spot where he executes the downed enemy on the left and aim at the wretch that is on the far right of the group that shows up which will have 10% hit chance when you target it without moving from the position he is at.
Aim with lancer and try to shoot at it. If you manage to kill it, it should pop. If not, reload checkpoint and try again till it does.