Walkthrough: Hidden Valley
Objectives:
The recommended course of action is to run past the sniper located to your left, staying within the shadows of the trees so as not to be detected (yes, I know the shadows aren't that large to begin with, but you'll have to manage).
Climb into the truck (after taking out the guard patrolling it and make sure to take the crossbow). This will take you to the next area. A series of black ninjas will remain in the area and will not advance until they've considered the trucks cargo to be safe. It may take some practice, but in case you're detected ? exit the truck before it reaches the guards. Then when it's about to take off, jump back in and you'll safely get past the guards without detection.