Id say hard rain is a good lvl to do it. The area you have to defend isnt actually that much enclosed, the whole campaign is also more open and for the 4th part u hardly even need to kill anything. U just run on rooftops etc, but for all campaigns, since ud be unlikely to attack the tank, u need 3 decent players.
I just did it tonight, in a pub, on Dead Center. The other players had no idea what I was doing. It was on easy of course, and two of them even got their achievement for surviving for the first time. It didn't seem that difficult, but I would imagine that more than one person trying to do it at once would become a burden.