When you get to Savanna, a church bell will start ringing and walkers will come from everywhere. While you are all running, Kenny will be grabbed. Shoot the walker in the head to save him.
Clementine and Ben will then be surrounded and Ben will run off so you have to save Clementine. You will have free aim of the gun at this point. Shoot the two walkers closest to you. Then start with the ones closest to Clementine.
You will get the achievement when you run away from the area.