This is a really tough mission. Here's how you do it.
Toss your rifle. Head to your right but on a diagonal forward. A steep hill will be on your leftside and you'll start to come over the top of a smaller hill right in front of the main compound. Put on your heat-vision goggles so you can see where the guard is who is patrolling the woods right in front of the main gate. Keep moving straight towards him then lay flat at the last minute. It'll take a bit of practice to figure out how close you can get to this guy without him seeing you. If he sees you then start over.
Once you're laying flat, that guard should be facing you. Take out your pistol and put two rounds into him. Get up and run straight to the edge of the woods by the corner of the fence. If you timed it right, one of the guards patrolling the fenced parking lot will be just walking by the corner of the fenced area. Lay flat and pluck him in the back.
Get up and pick up the sniper rifle and run to where the chopper lands. There's a tree there. Lay flat past the tree. You should be directly lined up with the catwalk on the right-side of the compound. Here's the trick. You have to be laying down in a position so that you can see through your scope to the catwalk on the OTHER SIDE directly behind the left front corner tower (between the left tower and the edge of the building there). Once lined up, make sure you hit the guard in the middle rightside catwalk when he is beyond the garage with all the guards inside. If you hit him too soon, the guard on the back roof of the main front building will see him go down and sound the alarm. Use the time while you are waiting for the middle guard to be in position to check your position through your scope so you can quickly drill the middle guard on the OTHER CATWALK just as he appears in the scope from behind the leftside building heading to the corner front tower.
Once in position, snipe both guards on the rightside catwak then immediately zoom in and wait for the other middle guard to come into view from behind the building on the leftside catwalk. This is a quick shot and you won't have lots of time. If you miss him then catch him when he turns and walks back. You have to take this guy out!
Now, toss the rifle and run straight to the leftside of the main gate and lay flat behind the fence near the trucks. You have to wait for the other guard patrolling the parking lot to head off away from your direction. Just as he turns away from looking directly at the front gate, crawl over to the gate and unlock it. Immediately crawl to your right along the fence and behind the truck parked right there. When you get by the truck's rear wheels you can stand up and run along the fence. About halfway down the fence, cut straight across the parking lot and lay flat along the leftside of the front door by the glass wall. Make sure that the outer camera didn't see you.
Get out your pistol and crouch near the glass window on the leftside of the front door. Stay out of sight of the outside camera. Lean and watch the outer camera. When the outer camera looks away, break the glass with your pistol and crouch walk through the opening and into the building.
Now what you are going to do here is to crouch walk backwards up the stairs and shoot out the camera right above your head. Keep an eye on the other camera. When the other camera turns away from you, proceed backwards up the stairs, take quick aim and shoot out the camera above your head. Then, quickly crawl the rest of the way up the stairs and get into the room with the computer BEFORE the other camera sees you.
Here comes a tough shot you have to make. Crouch by the end of the computer table (not too close to the window) and sit there and aim straight out the window onto the back roof. When the guard on the back roof walks past and turns and walks away from you, this is the perfect opportunity to pluck him square in the back of the head. Take aim and fire! You don't have much time. If you did it right, he goes down without making a sound and his body is out of range of the camera. If you miss and he stops then fire again. Chances are that you can hit him square in the head with a second shot before he sees you or makes noise.
Now, disable the alarm computer. Head back to the door but DON'T GO OUT THE DOOR YET! Lay flat behind the wall and lean around the door opening. You'll soon see directly across from you a guard come out of that door there. He'll look around then turn and go back in for a few moments. YOU HAVE TO KILL THIS GUY! When he turns and walks back into the room, crawl out into the opening of the doorway where you are and use your binoculars to quickly zoom into the side of his head. Align the picture of his head dead-center in your binoculars then quickly pull out your silenced pistol and fire! If you lined his head up right with your binoculars, your shot will be clean into his head. He'll collapse without making a sound. Killing both these guys with a single headshot is the toughest part of this mission.
Now, crouch walk down the stairs and down the other stairs. Open the door and go down the hall into the room where the safe is. Open the safe and take the documents. Crouchwalk back up the stairs and up the other stairs and get behind the door leading to the catwalk. Open the door and lay flat. Lean around the door to check on the guard patrolling the middle of the compound grounds. When he turns away or can't see you then crouch walk onto the catwalk. Here's the trick! Jump up onto the rail of the catwalk behind the door. Jump up onto the top edge of the open door. Jump from there onto the room of the main front building! Walk across the roof and jump down onto the catwalk on the other side (this is why you had to take out the two guards on this side of the catwalk).
Walk along the catwalk. When you get near the electric fence, crouch walk past the fence, hop up onto the rail just past the fence, and hop down to the ground nearby the garage with all the guards inside. They won't hear you.
Crouch walk along the fence (DON'T TOUCH THE FENCE!) and along the building until you get to a big column. Keep an eye on the guard patrolling the middle area of the compound. When the patrolling guard can't see you, crouch walk behind the big column and head directly to the door of the building. Get out your pistol and open the door. Crouch walk along the wall to the door. Get ready! You have to open the door then quickly shoot out the camera on the inside facing the door. This camera goes off fast so.. Open the door, aim and keep firing until that camera blows up. The camera right above your head won't see you and the two guards on the other side of the truck won't react.
Once the camera explodes, crouch walk in and head directly under the camera. Crawl the rest of the way along the frontwall and head straight towards the back stairs. Crawl up the stairs. Once in the room with all the computers, stand up and walk over to the switch and activate it. Walk back to the doorway then crawl down the stairs and head diagonally over to the row of machinery on the main floor. When you get behind the machinery, you can crouch walk along the machines without being seen or heard.
Get behind the machines and crouch jump onto the conveyor belt then immediately lay flat. Crawl directly to the back of the first scientist standing right there behind the glass wall. Don't crouch walk here! It alerts the other scientist. Break the first scientist's neck. Get out your silenced pistol and crawl backwards just past the end of the glass wall until you're in a position with a clear headshot into the side of the second scientist's head. Take the shot. He collapses and nobody knows anything.
Crawl over to the stairs and head up the stairs so you can upload the virus into their computer. At the top of the stairs, stand up and open the door. RUN to the computer. Get ready because all hell is going to break loose! Check your map computer to see where the guard patrolling the back catwalk is currently located. Get out one of your grenades!
Now, activate the computer and RUN out the door, down the stairs, and over to the conveyor belt. Activate the conveyor belt switch and jump onto the belt. IMMEDIATELY pull the pin on your grenade and when you leap off the side of the conveyor belt, toss it over the truck to where those two guards were standing and head straight towards the back door. If you do it right, the grenade will bounce off the back wall and take out the two guards behind the truck and you can make it quickly out the back door without being seen.
Get out the back door and run along the building to your left. Hopefully, the guard on the back catwalk is not patrolling right there when you exit the door. Get around the building and cut across the lot to where the ladder is along the wall (this is why you had to kill the guard patrolling this catwalk!). Before I learned that I had to kill this guy, I would find myself doing the whole mission only to climb the ladder with the guard standing right there waiting for me!
Quickly climb the ladder and jump straight over the wall.
Run into the woods to your left and keep just inside the edge of the trees so the guard in the corner tower doesn't see you. Circle around to the front of the compound and make a bee-line straight to where the rescue helicopter lands. Keep running. You'll run right past where you dropped your sniper rifle. If you're fast enough, you should get to the chopper just as it lands.
I did this mission in 11:35 without being seen, no damage, and 100% accuracy. It's a tough mission but can be done under the 12:00 time limit.