Co-op is excellent for resource grinding, between Kuban Energy, building materials, and finding recipes.
Otherwise, single player grinding is not too bad with the proper setup.
Level the Scout Sub Class. Get Scavenger, Fox Walk, Dex Super Boost, END Super Boost, and Stealth Camo. Unit Enhancement is excellent for placing co-op Machine Gun Turrets.
Set up a character with minimal gear equipped. Naked or lightweight DEX gear. A Melee weapon to break apart boxes and barrels. Have as many survival bags equipped as possible.
Go from transporter to transporter, picking up everything.
If you go over 100%, diving is the fastest method of moving.
If you go over 130%, attacking with a one handed melee is the fastest method of moving.
The Ruins have some nice goodies, as well as some of the random huts and buildings across Afghanistan and Africa.
Rinse, repeat.
To farm containers, rest in a tent to get requests from crew. Go to the container and open, hope for good stuff!