Once you enter this area, you will take control of a turret. You must cover for Murray as he makes his run for the key. First, the camera will pan over to the right, and Murray will be standing by a gate. Shoot the gate so Murray can begin running. Take out any guards that pop up along the way and watch out for the toxic barrels. Do not hit them or Murray will take significant damage. Once Murray's at the top, definitely look out for the machine gun toting guards. Once you take them out, Murray will grab the key and the mission is complete.