Enter the shack when ready to confront the stranger. After the scene go downstairs to talk with Willis. Exit the hut then wait until Jason calls Dennis. After getting your mission updated, head to the field. The game will suggest you to craft a fireproof syringe for added protection against fire.
There are five fields you need to set on fire. Remember to ignite the explosive barrels as well to help speed up the burning. After burning the fields, fight your way to the docks. You must use the RPG nearby to blow the boat up before it leaves the area. The mission will be complete afterward.