A strike from the outside of the leg with the foot works. Try it from the opponents side. He's facing forward. you are to his side--left, right, doesn't matter--and kick the side of the knee at the joint. Even if the leg doesn't snap at the knee, it will hurt him very badly. Then you can leave. Forget this move if he has a loaded gun.
Breaking the foot is much easier, but we'll leave that for another time.