You get this after dealing with the bandits when they attack the motel. Lilly will go off with a rifle to get to a better position while you stall the bandits.
When you get the chance, shoot and kill the fist bandit.
You are then given a rifle and need to clear out the bandits on the left side of the RV. Make sure they don't get too far round to the left.
You then need to do the same on the right side of the RV.
Once you have killed all the bandits, you have to save Katjaa and Duck when they get attacked by a walker.
You then have to kill all the walkers that approach on both sides. If you have Doug in your group he will call out which side they are on. If you have Carley, she will take the left side so you can stick the right side.
You'll get tha achievement when you all leave.