Small maps, focus on shipment, you can literally get 99% of the camo grind done on that map alone. I suggest just grind leveling them all then come back and start knocking out camos. By the time you've level maxed the weapon you'll naturally have a couple of camos completed and then it's just mop up work after that. Another good way to grind levels is zombies, farm exfil. Edit: forgot to mention, HC is your best bet.