Head to the Mountainside behind the Outpost. Look around on the Northern face and you will find a small hut with a Zipline running down to the camp. Ride it down and you will land on the roof of the warehouse in the camp. You can also get onto the roof of the warehouse by approaching it from the South and climbing up the vine that rests on that side. This puts you in a great position to quickly mark all of the 4 Soldiers and 3 Chargers that occupy the camp.