I created a video showing what I did to get the achievement. Basically just go through the level normally until you get to the barn. Once you get there, arm your survivors and then lure the walkers out and have your survivors kill them. Use the pistol to take a few out and use your melee weapon to help fight the walkers as they will be focused on your survivor. Take 3 with you at a time, each equipped with a melee weapon. If any lose more than half health, press B to make them stop following you, and pick up another survivor and arm them. Also make sure you do your jobs to keep the zombies out.