Awesome, I'll try this immediately!
EDIT: I did it! I managed to beat him in just three tries after this guide. Thank you.
I can add a small tip as well.
When the chopper arrives, you can run in real fast and cover behind the nearest steel plate on the circular railing until the chopper goes to the other side of the area. Then you can go back outside again where you just stood. The chopper won't return (it didn't for me, anyway), it just hovers on the other side, but it's still aiming at you, but you have plenty of walls for cover. If you're low on health at this point, take cover and aim closely to the wall so you can shoot the enemies right away when you see them coming on the radar mini-map.
When Trevelyan is all alone, wait for him to either cover, in which case you quickly strafe and shoot him a short bulletspray, or wait until he runs upstairs - When he's about to jump over the railing strafe out of cover and shoot him. You'll probably end up with only a few shots, but he won't have the time to shoot back at you. Rinse and repeat until you need to go to the elevator.
Note: This is crucial if you only have like 1/8 health left. Trevelyan will run a certain path at this point, but he can still shoot you. He will go up the stairs, turn right and jump down over the railing over and over again. If you are upstairs, the time to go for the elevator is right before he's about to jump over the railing. Trust me, you don't want to meet him in the middle of the stairs.
Jump over the top railing, sprint up the stairs, and crouch when you get to the upper floor again. Sprint the last few meters in to the elevator and push the friggin' button. Done!
Last edited by Demonshire; 11-04-2011 at 08:53 PM.