You get this after you escape from the meat locker. Pick up the mini cattle prod from somewhere at the back of the room, then talk to Kenny. He will walk out into the barn. You need to move forwards until you hide. After talking to Kenny in the stall, you need to lean towards the door to see where Danny has gone. You will be forced into a quick time event.
*** Spoiler - click to reveal ***As you peek out of the door a gun is pointed in your face. You have to very quickly grab it. Then when you get the chance use the mini cattle prod on him. You have to make a choice whether to kill Danny or spare him. Whichever you decide you will get the achievement ad leave the barn. Clementine will take note of your choice.