-( Story related, cannot be missed )-
In Graniny Gorki, as you're returning from the lab, you'll face off against the camouflaging COBRA Unit member known as The Fear. He will be playing in the trees like a monkey, or in his case, a spider, keeping to ranged attacks as much as he can. However, once you keep an eye on him and his trajectory, you can keep him in your sights even when he cloaks himself. This way, you can actually interrupt his attacks. With little time to respond, he'll just repeatedly jump from branch to branch as you shoot at him (like target practice shooting games).
Depleting enough of his stamina will force him to seek out ways of replenishing it. This forest is abundant in traps as well as fauna, and he'll shoot animals out of the trees and the brush. Once food spawns, he'll climb down a tree and head over to it to rest up. You can drop tainted food (spoiled or poisonous) to hurt him or you can beat him to the item before he can use it. Continue shooting at him while preventing him from recovering and you'll eventually beat him.
Should you have gone for the stamina depletion to kill him, you will be rewarded with the Fear's Spider camo (which will constantly drain your stamina but provides a large boost to the Camo Index compared to most this early).