This guide is meant for both Normal/Hard difficulties and, if done quickly, the mission can be done in more or less 2 minutes. If you have already got an A in the mission, things will be even easier but I'll explain how to do it without that advantage.

The main trick of this mission is to use wisely the M2000 Sniper rifle, which you can either get during the mission or have it from the beginning if you already have at least an A rank on the mission.

If you already have an A rank on the mission, grab the M2000 Sniper rifle in the cave to your right. If not, we will get it later.Then start running uphill and shoot the guard in the head with the Wu Silent Pistol.
Continue uphill without making much noise and lie down so that the guards in the car don't spot you. Then quickly shoot the guard to your left. Next, you have to take out the enemy who is coming towards you. Since you will have enough ammo, just shoot 4 tranquilizers if you don't want to stop to aim too much.

Originally posted by Big Boss:

PROTIP: enemies need 4 tranquilizers to the body in order to fall asleep. if you have enough ammo, don't bother getting the perfect headshot and just shoot 4 times.

Now you want to take out the enemy to your left, who is walking into the tents. If you are quick you may be able to get him before and, if not, you may have to follow him and tranq him so that he doesn't spot you later. Then get up the watchtower and tranq the guard near the truck to your right (Again, you will have more than enough ammo so you can just shoot him 4 times in the body).

IMPORTANT: If you already grabbed the Sniper rifle in the beginning you can proceed and eliminate the objectives. In case you don't have the rifle, you just have to get to the back of the truck near the last guard you killed and you will find one there. It should be easy and nobody should spot you. This can also be done before getting up the watchtower if you feel like it.

Get back to the watchtower, from where you can shoot both targets. You want to take out the bald one first, who is to the west of your position. If you are very fast you may be able to find him standing https://www.youtube.com/watch?v=7XmDYJBZZdc and talking to other soldiers. However, you will most likely find him already walking north by northwest (Should I include another pun in the same paragraph?). I'de recommend crouching after shooting the first objective so you don't get spotted in case enemies look to the watchtower. If you've done all the previous steps correctly, you should be able to shoot him in the head and not be seen. There will be an alert but they will not know it was you or where you are. Apart from that, the other objective will not know instantly. So you just have to look to your right, to the main tower in the Administration building, and shoot the black target in the head as well.

Now you just have to get off the tower and run north to the exit. When you are near, stop, since there will be an enemy stepping out of the booth. Tranq him and you are ready to go! Another possibility is calling the helicopter at the beginning of the mission so that you can just run back to the cave after killing the objectives.

I know that this is a very simple guide but I wanted to try the Steam Community Guides editor and I'll be happy if it helps someone. I plan on doing a Spanish translation as well. Leave a comment if you have any doubt or suggestion!