1) Turn around and get the bullet proof vest.
2) When your running up the walkway there will be a guy at the other end. You won't be able to see him but your gun will lock onto him. When you see your gun move up start shooting. Do this all while you are running. When you get to the top the guy will be dead. Grab his guns and turn on one. Don't use duel guns.
3) When you get to the top turn right and shoot the guy and grab his guns. Double back and kill the guys who are there. Get their guns.
4) Run to one of the planks and look down it. If there is a guy there shoot him and then run down. When you get to the bottom run down the stairs.
5) When you get down the stairs shoot anyone who is coming towards you.
6) Shoot the drone gun. a) You can run into the room and chase Trevelyan around and hope he goes into the bottom place where you kill him or b) you can take your chances and run up to the wall next to the door that leads to the computer terminal. Wait there and hopefully a guy will throw a gernade at you from the inside. Instead of it blowing you up it will bounce off the wall and kill Trevelyan and the terminal.
7) No matter which way you do it you have to go in and shoot the other drone gun.
8) If the gernade didn't blow up the computer terminal then you blow it up. Then run out and down the plank and start your chase with Trevelyan or just run straight to the place where you kill Trevelyan.
9) After you chase Trevelyan and he goes to the final battle. Drop down backwards and you will be facing Trevelyan when you land. When you land duck and start shooting at him. He will die.