When you begin the chapter, you have to go after Nadine, so make your way to the central tower along the linear path. After the last mud-slope, an armoured truck (ACP) will zoom by and your trusted jeep has been destroyed.
In this area, the ACP will be circling the ruins along with several rebels. Lockbox #13 is on the right-hand side.
Sneak around using the long grassy areas to reach the far end and duck underneath one of the platforms to find the orange box.
Now head back to your starting position and look to the left. Lockbox #14 is beneath a ruin directly next to the stairs that lead to the upper platform.
Once again, use the long grass as cover until you reach another orange box.
After the elephant sequence, run across the water past the herd towards the stone steps leading upward.
Lockbox #15 is at the top.