You can do this as early as the first level.
Just keep running around killing zombies with your fists and shotgun until the achievement unlocks. On level one you only have to worry about one kind of zombie, which is the weakest kind. There are plenty of chances to get this during other levels. You will have access to better weapons like the Chainsaw, Pistol & Deathray. When you encounter the zombie which are on fire, make sure the final blow is inflected by you, if the zombie dies by the fire it will not count as a kill.
This achievement will unlock as soon as you get 500 kills, so do not enter any gates as that will end the level.