First make sure you've at least gotten to the Answer The Phone level. Go to the hacking program from the main menu. Type cheat.exe and in the command box you see the word "cheat". Click it and type FFFFFFF1. Its a cheat for invisibility. Next go to the Answer The Phone level get to the part where the building is divided in two. You will see an agent jump in front of you, but can't see you because you are invisible. Jump to the side where he is standing, get behind him and keep on punching and kicking him. He will eventually fall down the gap in between the building and die.