Scatter Shot Trophy
As a team, kill 20 enemies in under 60 seconds in a Survivor match.
From the online menu go to friendly match > quick match> choose survivor mode in the ‘filter by game type’ menu. In this mode you will be ambushed by aliens upon aliens and your job here is to survive for as long as possible. To get the trophy just do as the description says, kill 20 enemies within 60 seconds with the rest of your teammates. I had 3 other players with me when I done this so it was pretty easy.