You must acquire the Chicken Cannon, a Legendary weapon. This is available from GebaN at the Cauldron of Chaos, or as a random drop.
Once you get it, start launching chickens at things.
Edit - I forgot to mention this, but thanks 'Chef Not Chief' for pointing this out:
Just a heads up, the chicken gun costs 70 demon souls from Geban. You can find those by luck, or by completing challenges in the Cauldron of Chaos, which are supposed to change daily or close to. So you better hope you get a random drop of the gun, or you're going to have to farm this one for a while. After beating the game and doing all normal challenges I had 5 demon souls. So the drop rate is pretty low.