This is the third story-related achievement you will get. After rescuing the hostage from the village, you will be tasked with meeting Prophet upriver near the entrance to a cave. This is a heavy jungle area with a trail that leads up and to the right, plus a small valley with a river running through it. There are many more KPA patrols if you follow the trail than if you take the lower, river passage. A good ways down the path on the right, a group of KPA will be fighting one of the aliens. There will also be a dead body here if you choose to look at it. You will also come across another group (snipers) a little farther down on the left, shooting across the river at the alien.
When you enter the gap on the left, you will get a checkpoint. You can also use the super jump over by the waterfall to the right of here. You can sprint from here. Once you meet up with Prophet, he'll admit he knew what was going on all along (the hostage was actually CIA and they'd been tracking Rosenthal for years). During this conversation, the achievement will pop. You can then enter the cave to complete the level.