Smee I got through. Spammed reload last checkpoint. hit them from range so objective marker was still showing. i used grenade launcher. once field was burning then reloaded checkpoint. After a few false starts it progressed to the next checkpoint.
I noticed when I was reloading checkpoints it would skip from 2 to 3 to 4 then back to 3...get it to 4 crops checkpoint, start your fire then reload checkpoint.
Might need a few tries, took me probably 10 minutes before it counted the fifth crop.