The Big Push
Defend your platform from the Locust and the Brumak. Try to use the Troika as much as possible.
At some point, the Locust will take over one of the platforms. Try to get rid of them with your Troika. You may also have to go down and help defending your own derrick from boarding.
After passing by the waterfall stand next to the Troika and wait for the window protecting the cockpit of the hijacked derrick to break. Quickly kill the driver of the hijacked platform.
After crossing the bridge you will have to defend the platform attacked by Brumaks. Aim at their mouths or the guns mounted on their backs. You don't have to worry about the Corpser.