On Day 6, you'll have to choose between hanging out with Gregg or Bea. Pick Gregg, and, after the "knife fight", you'll have a little fun shooting with a crossbow.
You can move the crosshair with
and, to shoot, you must hold
and release.
For this achievement, you need to shoot the target in the head. Not very difficult. The crosshair shakes a lot, but if you center it in the head you shouldn't have any trouble. The achievement unlocks the moment you get the headshot.
You have 8 bolts. If Mae says "I'm out of bolts" and you haven't unlocked the achievement, hold
to exit, and when you continue from the main menu, you'll be able to try again.