I have found a way to camp 90% of this map without being shot at, apart from running to the lever and back to the safe spot.
What you want to do is start the map and pick up some supplies from nearby. Climb the ladder onto the walkway, walk straight ahead and turn left, there's a pole, on the corner, none of the guys that run down can see you and you can just pick them off easily. Leave the commanders until the end and you can just climb down and headshot them both if they haven't already made an appearance by the last wave of enemies.
Next you need to climb into the room and activate the switch be sure to collect everything you need before you hit the switch.
Once you hit the switch, turn right around and go out the door to the rear of the room, on two occasions there were a couple of guys outside this door so be aware you may need to act quickly. Once done run straight back to the ladder and go back up to the safe spot until everyone is dead. It helps to memorise the route back to the ladder so you know exactly where you are going so you don't mess up under pressure.
Let me know if this helps any of you out.
Safe spot walkway
Safe spot on the map
Thanks to mande1brot for the pictures.