The key to the first part of this chapter and surviving the boss battle is eliminating Lazarević ranged attacks from his shotgun and grenades. Just keep as close to him as you can and he will just repeatedly pick you up and you can melee him. Do this at a spot where the tree sap grows and you will find that once you shoot the sap it will grow back after three or four melee attacks on Lazarević. He will occasionally throw you, just find a safe spot near some sap and wait for him to start the routine again.
This is also a good way of killing him on the harder difficulties.
After you kill Lazarević you still need to navigate the crumbling bridge and shoot the last guardian that attacks Chloe.
Trophy pops at the end the the cut scene.