In Black Wing go out of the spawn and in to the blue/green room with another Merc.
Turn left as soon as you get in and stand in the corner with your back to the railings.
Pull back and keep jumping at the same time with your team mate walking in to you (back and forth), you will soon go over the rail and be able to suprise all spys.
You can exit over in the corner and out in to the yard.
This can also be done on the ground floor in yellow/red but you will have to commit suicide to get out.