During the Mission 3: Uprising, you will start without a gun. Proceed through the tasks, until you acquire a new gun, and clear a few enemies. Proceed and make sure to turn off your flashlight (Up on the D-Pad). You will come to an area with a few enemies preoccupied with a chopper, with a trench in front of you, sprint down this trench, and take a right at the end, making sure to stay on the higher ground. Proceed to the car that is hanging on the edge, and wait for the 2 people to start talking. Once they do, Press B repeatedly until it falls, killing both of them and unlocking the achievement.
Please view the video below for a more detailed look at where this opportunity happens: