In the mission "Achilles Veil", when you meet up with Menendez after a shootout you'll have a choice to shoot either Harper or Menendez. You have to shoot Menendez for the achievement.
EDIT: To make it easier if you wait and do nothing for about 20 seconds Menendez will shoot you and the achievement will unlock. Shout out to RockstarELITEXX for the alternate solution.