I used to do this entire mission by sniping from the rafters but there was no way I could complete the mission in time unless I figured out a whole new approach. Well.. it took me quite awhile but here's how you do it.
When you start the mission, immediately crouchwalk TOWARDS the two guards by your position. They'll be turning and heading the other direction. Your job is to get quickly to the file cabinets, open up 3 of them, and get into the next room BEFORE these two guards come back patrolling near you. So.. act fast.
Once you've gone through the cabinets, crouch walk into the next room and through to the other side without stopping. Then, in the next area, crouch behind some boxes and use your heat-vision goggles to check where the guards are located. When they head back the other way, get by the entrance to the room with the control system and lean to take a look at the camera. When the camera is facing away from you, crouchwalk in and break the machine then immediately head back into the room you just came from. Get by the box by the opening near the stairs and watch the camera above the stairs.
When the camera looks away AND the guards aren't nearby, crouch walk out the opening and crouch jump over the railing of the stairs and down onto the steps below then lay flat and crawl over the boxes BEFORE the camera above the stairs sees you. This is very tricky because you have to break the machine, get out of the room, over the rail BEFORE either camera or the two guards spots you. Act quickly and decisively here.. Sometimes the two guards get startled but if they didn't see you then you can make it out of there before they make a fuss.
Once on the boxes at the bottom of the stairs, crouchwalk over to the wall opposite where all the guards are standing. Keep moving! Eventually, you'll get to the end and can crouch down onto the dirt road. Then RUN straight down the dirt road while keeping an eye on the guard further down by the gate. Get to the rightside of the fence door just out of sight of the camera. When the camera turns away from you, shot it out with 2 quick shots and unlock the fence door.
Get inside and grab the grenade on the table. Unlock the inner door then crouch walk through the airshaft to the end. You don't want to run down the airshaft because it gets all the guards in the rooms below running around crazy.
Once at the end of the airshaft, belly-flop onto the pipes and crawl to your right. You can get up and walk the rest of the way into the little shaft at the end. Go down the shaft and jump onto the big pipe below then crouch walk down the pipe until you can stand up and run. When you are able to stand, get up and RUN the rest of the way all along the pipe until you can jump off the pipe onto the big stacks of boxes near the end. Crouch jump down the rest of the boxes to the ground level.
Crouch walk up the stairs right there near your position and unlock the door. Don't run up the stairs because it alerts the guards in the dining area. Open the door then crouch walk in and head to your right. Crawl the rest of the way into the dining hall where the other guard is standing. Head straight for the door and unlock it. Go into the hallway and head straight down the middle opening ahead.
Crawl straight through the opening and into the area where all the technicians are standing on your left. Remember, there are guards on the metal catwalks and technicians standing around all over the place. Crawl along the cubicle walls heading straight for the big glass door ahead. Keep an eye out for the technicians. When you get to the end by the glass door, go to your right a bit then crawl up the stairs and hit the button which opens the big glass door. If you've done everything right here, nobody sees you.
Crawl through the door and get out one of your grenades! Crouch walk up to the computer. Grab the health syringe on the metal shelf right behind the computer.
Here's the trick.. Stand back from the computer and slowly approach it while crouching just until the computer-activation signal appears on your computer screen. You don't want to be too close to the computer because you want to be able to run fast without hitting the table the computer is on.
BEFORE YOU ACTIVATE THE COMPUTER.. have your grenade ready.. Here's what you have to do next.
As soon as you activate the computer.. hit your ESC Key to end the animation immediately and TURN TO YOUR LEFT AND RUN BEHIND THE GLASS PARTITION. If you did it right, you WILL BE SEEN ONCE but you moved so fast that the General wasn't able to get a shot off! This is one of the toughest aspects of this whole mission (getting away from the computer and off to the side behind the glass partition without getting shot or killed). Practice it a few times and you'll be able to do it 9 out of 10 times in a row.
Stand in the far corner behind the partition and face behind you looking at the back wall. Pull the pin on your grenade and throw it high so it goes straight through that door opened by the bunch of guards in that back room. This grenade toss is critical to completing this mission in record time because you have to kill all those guards in one explosion or they'll come up on your flank and kill you. Practice this grenade toss a few times. If the grenade goes through the door and explodes, you'll have taken out all those guards in that back room (including the one standing by the open door) in one big explosion. This is tough! If you miss the first grenade then immediately toss a second one but one or more guards will have made it out of the room by the time the second grenade blows up and you'll have to deal with them..
Once you've blown up all the guards trying to come out that back room door, immediately lay flat and crawl yourself in a position between the wall and the glass partition you are standing behind. Face forward again so you can see through the bottom panel of glass right at the other guards who will soon be coming through the big glass door to investigate. Don't worry if you're a little exposed to the General and his guards on the catwalk because they won't see you lying there. Don't worry about the General and his body guards just yet.
Now.. get out your silenced weapon and put it on single shot and watch as the big glass doors open up with guards standing on the other side. Fire THROUGH the glass panel 2 or 3 quick shots at each guard standing by the open door BEFORE they get into the area you are in. Once you kill the first guard, the others will stand there for a moment. This is your chance to pluck one or two other guards by the big glass door too. If any guards make it past the door and into your position then you're going to have to deal with each of them by firing through the glass partition as they approach your position from the other side. You should be able to kill all the guards coming through the big glass door before any of them make it more than a few steps into your area. Once you've got these guards out of the way.. now you can focus on the General.
Adjust your aim pluck the General's guard on the left with two quick shots into his chest. Or, you can use your binoculars to zoom in on his head and do a single head shot. Once he's out of the way, do the same thing with the guard on the right of the General. If you've done this right, both guards go down and then it's only you and the General.
Take out your binoculars and zoom right in on the General's head. Put a single round into his head and Mission Accomplished!
The first part of this mission is really tough because you have to keep moving and take some risks to get the machine disabled and down the stairs before cameras or guards see you. The last part of this mission is tough because you have to get away from the computer FAST! and then turn and do one super-accurate grenade toss or you've got problems. After you've killed the backroom guards, you then have to snipe all the other guards while looking through some glass. It's tough!
Anyway, I did this mission in 7:07 with 100% health and only seen once (when you activated the computer at the end of the mission).