The times which you can shoot him with a non explosive are when he is reloading, turned his back against you, still looking for you, or sometimes while shooting if you are lucky. The best gun for this is the shotgun. When you actually get a shot into him, it will knock him down.

CQC of any kind does not work on Null. If you try to punch, kick, or tackle him, he will slash you with his sword. He will also block 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 to kill or tranquilize him. Alternately, use any type of explosive, as he cannot block them.