Watch the World BurnG 30

To unlock this achievement you simply need to kill 25 zombies with incendiary shotguns. You will have to search through the world to find the shotgun. A good place to start will be at the gun shops. Once you have the shotgun you will need incendiary ammo which can also be found while you scavenge. You may also make incendiary ammo once you have an upgraded workshop, a library, and the proper resources.