Complete any co-op assault map over PlayStation Network.You must complete a co-op assault map over the PSN. This means that the trophy will not unlock if you do it locally.
You have a choice of playing in maps Mainframe, Systems or Cargo Hold. Simply finish a level with another player over the PSN and the trophy will unlock in the results screen. If you have trouble looking for a boosting partner, post in the Boosting Thread to find one.