This is a very straightforward mission with no surprises, but you must be very fast. The first time furrykef completed the mission (on easy difficulty), there was only one second on the clock! But if you keep moving, it probably won't come down to that.
Use the shotgun throughout the mission. There will be copious ammo along the way, so you won't run out if you don't shoot aimlessly. Just move, move, move. If you get knocked down a couple of times or get hit by a couple of grenades, it's no big deal. Do watch where you're going, and especially watch out for flamethrowers, but they're not any more of a hazard than usual.
By the way, the mission description says you must capture the spy alive. Just to be clear, you won't have to worry about shooting him accidentally. Bullets go straight past him.