Talk to Willis in his HQ then leave the hut. Next, head to the rice fields near the northern coast. Head to the marked location and sneak in to the marked hut. You can carefully slide down the cliffside or just use the zipline to reach it. After the scene, you have to take out the enemies before Rongo is killed. Before heading to his location, be careful of the mines planted in the rice fields. Talk to Rongo after reaching him.
Once done, follow Rongo as he makes his way to the village. Keep the enemies off him until he reaches his hut. He'll start searching for something so grab the ammo and health packs around and defend the area until Rongo finds the manifest. Talk to him afterward to snap a picture of it and to complete the mission.