To start off, make sure you finished the last mission with a silenced sniper rifle and a silenced pistol.
Go down the hill and head diagonally towards the main gain. When you get close, lay down behind some bush and wait for the guard on patrol by the main gate to walk behind the guard shack then snipe him clean through the head. If he makes any noise it alerts the two guards in the guard house. Then switch to your silenced pistol and crawl up over to the road and position yourself about 20 feet from the shack window so you can see directly into the window. Get ready! You have to pluck the one guard clear through the head and then blow the brains out of the second guard as he stops momentarily to open the door to exit the guard house and run to turn on the alarm Your aim has to be perfect. If you miss then keep firing. I've missed this guy by the door but get him clean in the head on the second or third shot. If he exits the guardhouse then you have to kill him before he reaches the alarm. Next, shoot out the camera. Get up and run to the gate and unlockit. If you do this right with 3 clean headshots, you'll be in the front gate in under a minute.
Next, run around the back of the guardshack to the left and get in position to snipe the guard in the tower. Another headshot else the guards in the building below him will be notified. Then, snipe the guard working outside the guardshack by the fuel pump. Then, snipe the guard in the guard shack through the window. Shoot out the camera while your at it. That is a total of 4 shots.
Now, skip disabling the radio. You don't need to. Run directly towards the right of the wall of the building across the road. Use your map computer to make sure the guards patroling the field to your right are walking AWAY from you. You should be able to run straight across the road, along the wall, and around the back at full speed. Run around behind the camera over the back door and shoot it out. Then, kneel down and unlock the door. Crawl in and then kneel behind the boxes right there to your right. Crouch jump all the way to the metal second floor. Use your silence pistol to shoot the guard up there with you straight through the side of the head. He collapses without making any noise.
Crawl over to the ammunition and pick up what you need. Be sure to pick up 2 land mines. Crawl back down to the boxes, fall down to the ground floor and head toward the back door. Check your map computer to make sure the guards outside are walking AWAY from you! When they are, get out the door and run all the way back the way you came along the wall, over the road and past the helicopter to the fuel pump gate near the guard shack. Unlock the gate, turn on the pump. Then go back around to where the pump itself is. Before turning on the pump, plant one of your mines off to the right of the pump. Then turn on the pump and quickly plant a mine right there by the entrance to the pump. Get out of there fast!
Run along the front fence back to the main entrance and outside the compound. When heading out the front entrance, go around to the left side of the fence and use your binoculars to find a position where you can lay down looking straight into the entrance of the fuel pump which the guards will try to turn off. Lay down with your sniper rifle and keep looking through your scope directly at the fuel pump. The mine will take the first guard out who tries to shutoff the fuel. After that, it's your job to snipe anybody who goes to turn off the pump.
Make sure you're hiding in a bush because the noise of your rifle (if you fire it) will cause the guards in the field on patrol to come looking your way. Wait for the clock to run out and the mission is over.
If you've done it right, you were never seen, your health is 100% and your accuracy is 100%. And, you beat the clock! I finished this mission in 8:05.