In act 3, you'll be defending the school from walkers. There are 2 waves. Watch both sides, since they can sneak up on you.
The first wave has 5 zombies. Headshot them all. You don't have be perfect, but they all have to be head shots. If one catches you, let them kill you and you'll restart.
After the first wave, the game will save. There are 6 more zombies. Again, if one catches you, let them kill you and you'll restart.
The achievement will pop shortly into the cut scene right afterwards, after everyone has come out of hiding.