Completed this one by myself in a local game. Very tricky. I was a Marksman with an M107 sniper rifle.
Objective 1 - Eliminate Guard Post: Run up far enough on the right side that you can snipe the 50-cal machine gunner before they know you are there. You will take out the rifleman a little bit later to complete this objective.
Secondary Objective - Destroy Supply Truck: The first guy you shoot or the first time you are shot at will start this truck fleeing from behind the houses and you must move very quickly to get this objective. Should be much easier to take out all the guys and the vehicle if you have a large group, but here is how I did it by myself - immediately after sniping the 50-cal machine gunner at the guard post, I switched to a smoke grenade and tossed it in the corner of the cliff where grid squares F4 and F5 meet. From the smoke concealment, I quickly tossed a grenade up toward the road where the truck was fleeing. As long as my grenade got close enough to the truck (looked like 10-20 ft. from the drone view), the truck would stop and stay put for good (I successfully did this twice - died in the caves the first try...grrr - be vigilant as it may take a few tries if you're attempting it this way). Once I cleared out enough guys, including the marksman by the electrical tower, I could snipe the truck with the M107 at my leisure. Objective complete. You most likely have the Guard Post objective complete at this point as well.
Objective 2 - Eliminate Nearby Patrols: This part of the mission is fairly easy as long as you're patient. Scout ahead with the drone and take out enemy patrols. Watch for the VBL that drives down the road where you destroyed the supply truck. I think I got a glitch because even when I finished the mission, there was still a red oval at the left side of the map. I had scanned the entire map with the drone and found no enemies.
Objective 3 - Destroy Weapon Cache: This is up in the caves. I recommend throwing a grenade or two at either cave entrance to see if you can get the rebels to come out and die on your terms instead of trying to go in the caves after them. Plant the charge and ... boom!
Objective 4 - Capture Roberto Duarte: If you destroyed the supply truck for the secondary objective, Roberto will be at the far right camp on the map (the one with the fire pit and a bunch of tents). Take out everyone around him and walk up to him so he follows you. *Note - if you didn't destroy the supply truck, you won't get this achievement and the truck will pick Roberto up and bring him to the caves.
Objective 5 - Extract with Roberto Duarte: Once Roberto is with you, scout ahead with your drone as more rebels should have spawned by the houses and the road where you destroyed the supply truck and also around the bottom of the cliff where F4 and F5 grid squares meet on the map. Take out everyone in your way and get back to the helicopter where you originally started the mission.