I did this mission solo on normal. It could be done on any difficulty with 1-4 amount of players. I recommend at least having the "transporter" skill under the enforcer tree so you could move the weapons faster since they're heavy.
On day 1 you'll have to find the warehouse containing the weapons. All of the weapons are located in this one warehouse, but make sure you thoroughly search it so you don't miss any. The two white trucks always have weapons in it, and there are usually a few cases around the trucks on the floor or tables, and there also may be some upstairs.
If you're doing this solo you may want to take the weapons out of the cases and delivering them to the truck one by one so the cops don't start taking the guns you bagged while you're delivering one. Once you deliver the last weapon, Bain will let you know that you cleaned the warehouse out. The achievement will pop right when he says this.