PLEASE NOTE: The online servers have been shut down since April 2014. Trophy is no longer attainable.Well, just as it says, complete 5 missions with a full party of 8 in the Cooperative campaign.
Great, with the servers going down at the end of March 2014 and players scarce, you would think this impossible. But, you can play the same mission five times and you can have second controllers count as one of the eight. Big help as now you only need four players minimum to pop the trophy and not eight. Of course, more players wouldn't hurt.
In our case, we brought in second PSN accounts. It might be possible to simply have a guest but this is untested. PSN accounts are free and we know they count toward the eight. Also, since you can play the same mission over and over, the Chicago map is open right out of the box - no need to open additional maps or missions.
Set up the Co-Op lobby to allow late entrants (with a password). This way, if someone drops from the group, you can still bring in another controller. Although not required for the whole mission, eight controllers must be present at the end when the Mission Success screen appears.
We had two soldiers each with HVAP Wraiths, one Medic and one SpecOps. We pretty much left our other controllers where they spawned. No one ever died. This method popped the trophy for
HamsterExAstris,
ImperialPie,
NastyNeezer and
NonKosherSalt