At the end of every multiplayer game played "Boasts" are awarded to players who fulfill their requirements. There are 6 boasts awarded: 1st Strike, Assists, vanquishes, heals, Revives and vanquish streak.
To unlock this trophy you need to end a multiplayer match with the most assists. I would recommend staying with your team, spraying a lot of shots into the enemies and hope that your team can finish them off for you.