How do you defeat null in Metal Gear Solid

If you mean Gray Fox from Metal Gear Solid 1, then you can throw chaff grenades. These disrupt his electronic parts and allow you to hit him without him fighting back for a few seconds.

When you run out of chaff, just run in circles around the center of the room. While he's chasing you, suddenly turn around and throw a few punches. When he puts cloak on, either look for his shadow, or turn on your thermal goggles to see him and hit him before he hits you.

When he starts "teleporting" from one spot to another near the end of the fight, don't attack until he does a slow, powerful punch. He leaves his sides open at this moment.