You can do this early in the game after you defeat the rat in the bar and find yourself wandering around Houton. There are a number of chickens in the town. If you get close to one you can kill it with your sword. As you wander around, more chickens will appear so keep killing them (you'll occasionally get a small item for killing one). Once you've reached 30, the achievement will unlock and an enemy chicken will appear who will attack you if you get to close.