Now that everything is set for your infiltration of Lazarevic's camp, you can get in there and hopefully find a clue on where to go next. You'll get to experience a heavy firefight on this chapter, as well digging around in some forgotten ruins. Remember to take cover and move from cover to cover during combat. You'll also need to make sure your friend doesn't get too overwhelmed, because if Sully dies you'll have to start over at the last checkpoint.
Assault on the main compound
Start out by following the large, black pipe through the jungle. It leads you right to the main camp where you'll first see Lazarevic in a cut scene. When Sully blows the charges, it will create a distraction, and only a few guards will remain behind. From the cover you start at, stealth attack the guard that walks up, then sneak up to cover behind the guard directly in front when the one in black walks away. Take him out too, and pick up the guns there. There are two guards remaining in front of the main building. If you want to, you can sneak around to the right (as you're facing the main bungalow) and stealth attack the other guys. Whether you use stealth or just shoot them up, once they're taken out and you approach the building, you'll go in and find Marco Polo's journals.
When the bad guys get back, you'll be right next to a never-ending supply of grenades. Take them with and throw them with . You can throw them while aiming with , or for more accuracy you can hold to show your trajectory and move it around with . This is a good place to get some of the dynamite multi-kill trophies for taking out two, three or four guys with one grenade. You can stay in Lazarevic's shack for a while just lobbing grenades, but eventually you'll need to take some guys out with your regular weapons and they tend to throw grenades in at you from time to time. When you see a grenade come close to you, quickly dive away with .
Just move from cover to cover by the shack and take the guys out as they come. There are a lot of them, and at one point Victor starts to get pinned down. You'll want to take out the guys up on the ledge on the left, then move to the right side and take out his aggressors there afterward. Once all the ground troops have been disposed of, the machine gunners up in the tower will start nailing you and Sully. Grab some grenades if you're out, and move over to cover right by the broken down ship below the tower. Using , adjust your arc so you can throw a grenade right into the tower window. The machine gunner on the side you're on may lay down too much fire for you to get an accurate throw off in time. In that case, just take him out with a gun before lobbing a grenade inside. If you haven't already gotten it by now, and you're able to make the throw without taking out one of the gunners, this is a really easy way to get the Double Dyno-Might! trophy.
To the ruins
While you're over by the ship, you'll want to shoot down a treasure from the crow's nest at the top of mast if wasn't knocked down already. You can see it from behind the ship, on the side closest to the building with Lazarevic's things in it. Grab the 020Mongol Paiza Passport, then take the fallen timber up to the cliff from that same wrecked ship. When you're up there, take a left and go down the path farther into the jungle. After jumping down a ledge, you'll start going uphill, climbing ledges until you break out into the sunlight. Just keep going, following the steps and path until you meet up with Chloe.
Head into the dark path, and around to the left you can drop down into a hole in the ruins. When you get down there, look for a giant statue's head with a gaping mouth in the middle of the floor. Above and behind it, you should see a treasure glowing up on the wall. Shoot it down from the statue's arm to receive the 021Jade Belt Slide. Right across from the big head is a doorway with bats that come flying out when you enter. Inside you'll find a piece of resin to the left on a barrel, and you can pick it up with .
Once the resin's burning, the UV rays reveal bloodstains all over the place. Follow them back out through the doorway you came in. You can either follow the trail all over the cavern, or just head straight across, past the giant head to the opposite doorway. In there you'll find the trail ends and you'll need to press , then to move the stones out of the way. Continue straight through the revealed path to a skeleton at the far end of the room you come to. Once you find the golden passport and the map showing you where you're going next, look up to the left on the wall (this would be to the right of the skeleton, if you were facing it) and shoot it down. Pick up the 022Yuan Dynasty Helmet treasure, then follow Chloe back out of the cavern.
You'll meet Lazarevic's guards when you get out, and you'll need to escape. Just run down the path, diving, jumping and climbing as needed. You'll run into a guy right away, who you can fight, and after him when you try to jump a gap you'll fall down and need to climb back up to the cliff top. After that you'll reach a dead end and you'll jump down to the water with Sully to get away from the bad guys.