You will find the stranded camp; Marcus will send them to a safe place together with Cole and bard. You and Bard will search for a route to Nexus.
Going along the lake you will bump into a Theron using a crossbow with exploding quarrels. Don't approach him, but try to eliminate him from a distance. When only he starts to target, hide. You have to look out, because a single hit ends your game. After killing the Theron, take his crossbow.
After that you will arrive at some ancient ruins. A Locust gunboat will appear, but for now concentrate on the enemies on land. You will find the Stranded's Journal: Stu.
You will receive and order to immobilize the gunship. In order to do that, target the yellow containers and shoot them. They are at the front of the gunship, next to the Troika.
Use the destroyed gunship as a bridge and defeat the Bloodmount
Your target is the boat, but before you reach it you will have to fight with some more Locusts. Especially look out for Therons with crossbows.
After taking a look at the boat, a cutter will destroy the pier that you're standing on. Defend this raft from the attacking cutter.
Get on the enemy cutter and shoot the remaining enemies with the Troika.