You will need to get the Revolutionary Fighter Achievement first. You get this by collecting the 3 signs in each level for a total of 45. This gives you the knife.

Go into the chapter selection and go to Chapter 15. Switch to your knife and hop down into the room where you will fight Brno, and run to the right. The knife has no ammo, so don't worry about that, but press the button for a health pack just in case you need it later on.

Eventually you'll run into Brno and the fight will start. Don't focus on hitting him yet, you want to get him to chase you to the right. Eventually you'll run around behind Brno, and you can start stabbing him at the spot shown below.



If you get him stuck up against the ledge you can just stab him continuously and he can't escape (his shields don't do anything against the knife). Also, the reinforcements can't hit you and will just jump in front of your knife and die as well. Once you get to this point you can just click repeatedly until you win.