Stand next to the anchor post on the stream bank near the day camp. Shoot a rope arrow into the rope-wrapped tree trunk on the ledge ahead. Climb the rope to reach that ledge. Then move up and around to the left.

As you approach the top of the slope, you'll hear 3 Solarii scouts bantering down below. Eavesdrop on their conversation as you pick up the GPS Cache (12/15) near the edge on the right.

When the men are done talking, one of them will move off to the other end of the ledge where they're standing and face away from the other two. This will make it fairly easy for you to kill him with a bow-and-arrow headshot without attracting attention.

Notice how he glows white in Survival Instinct view. Unlike his red companions, he will not alert the others if you can kill him quickly.

Then you can take out one of the others the same way...

...and hopefully finish the third before he can do too much damage.

When the coast is clear, jump onto the top of the painted wooden archway. Walk across and jump onto the ledge with the salvage crate and arrows. Climb the steps to light the statue (9/10) for the Illumination Challenge.

Drop down to the rocky path and head down toward the scouts' camp. Jump across the gap in the path and loot the bodies where they fell.