This achievement is unlocked during the "Dark Water" mission -- specifically near the beginning, where you are going up a staircase lead by Graves and Alejandro. As you're going up the stairs, they will mostly be taking out enemies for you. After reaching the top of the staircase, Graves will hear an enemy inside of the first porta-potty and shoot at him through the porta-potty door. To unlock the achievement, simply kill the enemy before Graves does.