Welcome Party (Baron Lumber Mill)
After liberating the Mill, talk to the starred prepper to get this side mission.
You now have 30 seconds to ambush a cult convoy out front. Note that your only goal is to destroy the large box trucks- destroying other vehicles and personnel counts for nothing in this mission. There are a number of ways to tackle this, but the easiest is to man the gun on the turret-truck by the tracks here.
Just pour on the fire as the convoy appears, and you'll have mission completion in no time flat.