Take a bike to the farm and ride to the back of the farm using an indirect route, bypassing the survivalists in the fields and tractors. Quickly steal the harvester, then run over the survivalists that are on foot between you and the road back. Try to overturn the pickups that are in your way. If they are still following you on the road, do a drive by when one is next to you to kill the gunman on the truck bed. The Harvester can take quite a bit of damage. Hit the pickup until it catches on fire, then get away.