Going off Dredds idea perhaps 2 4man squads are inserted into a skyscraper - one from the ground and one from the roof and you have different tasks to complete (escort hostages or turn power on to the lifts etc etc) before meeting up to tackle the final boss. It would my expectation that comms would work for each team and then finally the whole squad at the end.
This could be taken asynchronously where you matchmake for the final boss with another team who has got there in the same time rather than potentially waiting for the other team you started with to get there.
Hell a fail state could ignore the loss of a team to make the challenge even harder...
ps3ud0