This is just about unmissable throughout a normal playthrough of the campaign. In the mission "No Hope For Hadley," you will be told to take a smart gun and go with O'Neal. The smart gun will auto target xenos. Simply kill ten enemies and the achievement will pop right away.
The smart gun will run out of ammo eventually, which you can keep track of by the % bar on the right side of your HUD. If SOMEHOW you don't get ten kills, just reload the checkpoint. Easy 20G.