-Shoot him with a non explosive when he is reloading, has his back against you, is still looking for you, or while he shooting (if you are good). The best gun is the shotgun. When you actually get a shot into him, it will knock him down.
-CQC of any kind is useless on Null. If you try to punch, kick, or tackle him, he will slice you with his sword. He also blocks any bullets you shoot at him. The only way to defeat him is to hit him in the back or wait until he reloads his gun. He fires three bursts before having to reload, and will shoot even if he cannot see you. Wait until the third burst, then quickly jump out and shoot or tranquilize him. Alternately, use any type of explosive, as he cannot block them.