For this trophy you will want to have 2 - 4 players. The best method is to split into teas of 2 on each end of the bridge, having a bot - human combo or a human - human combo. The reason for this is if you are all on one side, should they spawn on the other side they will freely run to the bridge, and enemies that spawn on your side will do the same (mostly Shields and Tazers do this) to avoid you from getting this without splitting up. Once the chopper arrives, there will be a small delay but it will pop should no one touch the bridge. It is highly recommended you bring medium to heavy armor (unless you don't have it unlocked), since you will be down 2 as it is.