Here's how to do it cleanly, and beat the clock!
Run down the little hill and over to the road to the right. Cross the road and lay down in the bushes and look directly through your scope at the fence directly in front of you. There is a guard right there walking along the fence. Snipe him clean through with the rifle that makes a big bang. This causes the guard in the guardshack to come out the door. Snipe him clean through. Shoot the guard in the tower by the main gate. Here's a trick: Immediately fire a single shot into the door of the guardshack by the inner gate. This gets him to open the door for you. Now, get up and run into the maingate guardhouse and use the computer to unlock the gate. Run out the door and slightly off to the right then head straight for the inner guardshack. Don't worry about the vehicle. It won't notice you. I've even tried jumping on top of it.
Use the vehicle to shield the guard from seeing you while you run toward his guardshack. Get up as close as you can to the inner guard shack. Eventually, you'll have to lay down. Let the vehicle pass by and the guard should have stopped running in and out of the door. Crawl up close and silently shoot him in the head with a clean shot. This shot is critical!
Stand up and run into his shack and disable the computer. Run out the door and around under the camera into the inner compound and along the fence to your right then crouch walk the rest of the way to the corner by the camera. Use your heatvision goggles to view the guard inside. Lean around and keep an eye on the guard walking around the truck in the MIDDLE of the building. As soon as that guard passes behind the truck in the middle of the building and starts walking toward the front of the truck again, crouch walk right under the camera, past end truck, and get underneath the middle truck as quickly as you can then lay down. Take a breath but you must act quickly.
Now, you have to time it so that the guard who is walking back and forth between the buildings has turned and is again heading back OUT of the building you are in, AND the guard walking around the truck you are hiding under is heading toward the back of the truck, AND the guard over by the back door is facing away from you. Then, you have to walk directly over quietly toward the back door. You can walk part of the way to make some distance but eventually you'll have to crouch-walk the rest of the way to remain silent. Open and go out the back door, close it behind you and take a breath.
If you timed it right, you can literally go from being outside the building crouching by the corner camera to the back door with only a slight pause while your are under the middle truck. You can be in and out of that building in under a minute. Acting quickly and decisely is the key to getting through that building FAST!
Once you are outside the back door, run over to the wall and partially along the wall before diving to a crawl and crawling the entire rest of the way over to the back gate. You can stand and run the last 40 feet or so. Unlock the back gate and your are into the area where the helicopter is. I've been past this gate under 5 minutes.
Once into that area, you need to snipe the guard on the wall tower. Crouch walk along the edge of the building then crawl the last bit while looking through your binoculars in zoom-mode. Stay focused on the guard. Look directly at the guard and crawl yourself in a position where the side of his head looks like a full-size poster in your binoculars. Then, silently shoot him directly through the head. He collapses without a sound.
Now, get up and run to the corner of the building you are laying besides. Stand by the bottom of the stairs and lean around to watch when the camera faces away from you. Then RUN up the stairs and open the door before the camera swings back around. Disable the machine and then go through the back office door. Take out your knife and break the glass. Crouch-jump up through the window down onto the ground.
Run underneath the second building and head to the side by the bottom of those stairs. Stand there and lean around until that camera is facing away from you. Run up the stairs and open the door. Head into the back room and BEFORE you disable the machine, use your knife to break the window by the machine. Crouch and disable the machine. Then crouch-jump out the broken window down onto the ground. Crouch-walk over near the wall. Shoot the guard three shots in the back.
Stand up and run THROUGH the compound gate and head directly over to the leftside of the frontdoors out of camera view. Crouch-walk the rest of the way and use your knife to break the window. Before you crouch-jump through the window, use your heatvision goggles to see if either of the technicians in the two rooms have noticed anything. If they have then wait. Otherwise, crouch-jump through the broken window.
You MUST kill the technician guarding the briefcase (Left door) first. Crouch walk and open his door. Don't shoot. Break his neck! Grab the briefcase and get ready for some fast action!
Open the second door (closest to the front door) and get as far into the room behind the door as you can without the camera seeing you. Fire a single shot into the back of the guard's head and then IMMEDIATELY swing around the door and fire 3 quick shots into the camera while it is still blinking red but hasn't yet sounded the alarm. You can do this. Kill the guard then immediately blast the camera. It's easy after awhile. Once you've done this, disable the computer.
Crawl over to where the control room is. Wait for the guards to be turned away and crawl down the stairs then immediately underneath the stairs. Go into the room off to the right and break that technician's neck. Disable the computer. If you fire a shot or make any noise the guards will come to investigate. Crawl back out along the back wall and do the same thing to the technician in the other room. Disable the computer.
Now, crawl along the back wall and get underneath the stairs again. Wait for the guards to be turned away then quickly crawl back up the stairs and over to either side door. Open the door. Lean around and wait for the guard to be looking away from you. Crawl up the ladder.
Now is the tricky part. You'll probably be seen just as you're sliding down the cable. Sometimes they see you and sometimes they don't. Two tricks.
First, crawl over to the ledge on one side and peak over until the guard walks by underneath you. Shoot him in the head. Crawl over to the ledge on the other side and peak over the ledge until that guard walks underneath you. Shoot him in the head. These are very tricky shots and you have to time it perfectly. If you do it right then both guards collapse without a sound. Then, crouch-walk over to the ladder. Climb the ladder and slide down the cable. Mission Over.
Second, what I've done is after I've gotten the briefcase and shot-out the camera, I've crawled down the stairs and shot the technician at the bottom of the stairs in the back of the head. The two guards above will come running but they haven't seen you yet. Before they reach you, turn around and lay flat by the dead technician with your gun on full-auto aiming at the top of the stairs. As each guard gets to the top of the stairs, blast him before he sees you. If you do it right, you kill both guards at the top of the stairs and the two technicians in the side-rooms never heard a thing! Once you've gotten the two guards and the technician at the bottom of the stairs all out of the way, you can shoot the two side-technicians quickly and efficiently without having to worry about noise. Then, you can run upstairs, climb the ladder, and slide down the cable without worrying about a thing.
If you did everything right, you'll have so much time remaining that I've even gone back and killed everybody and still got down the cable in time.