Sneak up on him and shoot him once. Then, run off and get behind a rock on top of risen ground in the middle of the arena. When he is circling the rock your are behind, you should also circle as well. He will eventually stop and yell at you to come out. He should pace back and forth slowly while you hide behind your rock. When he stops and is not facing you, shoot him in the head. Perform this until the point that he becomes too smart to fall for it again. By then his health should be low enough so that you can run out and keep shooting while strafing. After you have gotten his health down all the way there will be a intermission sequence. Look at your compass at the bottom of the screen. He is the red dot. Find him and he will run at you. Run, but keep firing at him with your pistols and he will blow up.