This mission i found easy, but i did have full health, hitman rating on most guns, and tooled up to fuck, but...
Right, try n snipe as many people as you can, saves you engaging them properly, but DONT worry about getting spotted, its not ideal, but i did EVERY time
Once you spotted just enter the bunker via the airvent in the floor, its well easy to do it in time without even taking a single hit.
Once you inside what time it is dont matter, just go thru the building killing everyone, but theres a room, not marked, with the SAM site controls, find that too, a fair way in, just off the sorta "main" room where theres loadsa machinery.
Once you near the end there a big spiral staircase to go down and it quite hard to not get too shot up, just be careful n quick with the guns, or throw lots of grenades down.
Once you at the bottom get the jetpack on, and fly up up up, dont stop of slow down till your well high away from their weapons, and as long as u turned the SAM sites off youl be ok.