One of the easiest opportunities to get this is in the sixth mission (Dust District). After you get given the option to choose between the Howlers and Overseers, head right toward the Overseer part of the city. Just before actually entering the Overseer area, climb/blink up onto the balconies on your right and sneak around the corner. You'll come to a place where you can see a pair of Overseers preparing to execute a pair of prisoners. Use Domino to link the executioner (with the pistol) to one of the prisoners and the achievement will unlock after he pulls the trigger.
If you don't want to invest in Domino, you can always save, buy the Domino upgrade, get the achievement, and reload.