This mission can be done in record time without firing a single shot. Here's how you do it.
Run straight down the side of the mountain towards the first missile launcher. Be careful not to get hurt running down the hill. When you get near the bottom, crouch walk then crawl to a position near the boxes and get yourself just in range of your laser designator. Aim the designator at the first launcher and wait for the pilot to acknowledge that he's got a lock on the target. Then, get the hell out of there!
Get up and run to your left up into the trees on the hill and then around the next hill and head straight down the mountain side. It may take a bit of practice to figure out just where to run without some of the patrolling guards seeing you. You can do it though!
Run down the hillside towards the road below and cross over the road. Run along the edge of the cliff above the water and head straight to where the other two guards are patrolling down by the water's edge. You can bypass all the other guards patrolling the road. When you get down by the water, lay flat and wait for the guard near the water to be walking away from the water's edge. This too may take some practice but you can get past both these two guards, along the water's edge, and up the other side without being seen. If you get to the water's edge and the guards isn't there yet then take the initiative and try to get past him BEFORE he get's to the water's edge. You can crouch walk part of the way.
Work your way up the hill and get yourself into position so you can see the second launcher. Lay flat and aim your laser designator at the second launcher. Wait for a lock. Then, crawl backwards a bit to get out of sight and run straight up the hill heading directly towards the drop zone. Be careful here because it's tough to get out of there without one of the guards spotting. And, it'll take a few tries before you know just how to run straight up that steep hillside without falling backwards or being seen.
Once you get to the top of the hill behind that launch site, you should see the cargo plane actually making the airdrop. Head straight in that direction. Check your map computer to make sure that you aren't heading straight into a patrol of guards. Keep moving! When the guards leave the drop-zone area, run up to the smoke and grab your stuff. Mission accomplished.
I did this mission in 4:57 without being seen, no damage, and I didn't fire a single shot. Quick and easy but it takes a bit of practice to learn how to run up and down those hills without getting damaged. And, it takes a few tries to get in and out of a target area without being seen while you try to run away.