This is a rather easy first mission, the only real worry is the few enemies at the first post that could reveal your location, and knock that S rank off.

Things to keep in mind with this mission

Do not kill anyone besides the targets, killing others will automaticlly lower your rank
Do not get caught by anyone else or use Reflex, doing so will also lower your score

Keep these in mind, and you can practically run though the entire level.

There are 2 targets for this mission, one on the tall watch tower to the north east, and one to the north near the refuge camp. You can take care of both with a sniper located at a nearby truck and from a single watch tower location.

To see your targets and their general location, open up the map and look at their faces, The bald man is at the refuge camp, while Mr sunglasses is in the watch tower. Its reccomended to take out the bald man first as he is the hardest to hit, as well as his location changes after 4-5 minutes.

After taking out the first guard, crawl on the small cliff to avoid the jeep and quickly take out the guard. Make sure to take out the guard in the left camp as well as the one walking down the other cliff near the bridge.

Afterwards walk up behind the truck and take out the next guard, otherwise if you jump into the truck, you will be seen by the guard. Jump out of the truck and run back to the watch tower near the third guard you knocked out near the camp. Climb the tower and Open your map to call a chopper to the starting point of this mission. This will save you time and nab that S rank.

Now aim your scope at the blue fence, being careful that the other guard doesnt see you. after a minute you should see your target with a few other guards, take your targe out quickly, to reduce the chance of being seen.

After target B is down, turn to your right and aim at the center window in the tower. Zoom in all the way to see target A Smoking. Take him out with a single bullet. The second kill or shot will set off an alarm, so make your shots count.

Afterwards Head back to the chopper to complete the mission. So long as you follow the above steps, youll Get an Easy S rank

See the video below to see this plan in motion.