Kill 2 Helghast with 1 Double Shot from the M13 Semi-Automatic Shotgun (1)
You will come across your very first M13 Semi-Automatic Shotgun during the first level of the chapter Vekta Evacuates. Midway through the level, in the room with blood in its doorway and where three ISA soldiers are holding perimeters, quickly run over to the other side of the room and into the small cell where an ISA soldier lies dead. Right in front of him, you will see the shotgun that pertains to this trophy.

Do not kill the six Helghasts that come storming through the door. You will need them to earn the actual trophy. After you've picked up the Shotgun, try and line two of those enemy soldiers perfectly in a straight line and press the button (Default) to unload the devastating double-shot secondary fire. The trophy popped almost a full minute after I got my double kill and about a few seconds after I cleared the whole room, so be patient before you start worrying.