You can get this earliest in Chapter 2, level 1.
After you get the first "Checkpoint Reached" message of the level, keep going for a bit until you come to a room with a staircase on the left leading up, and a door to the right on your level.
Go to the door and go inside. You can find the Pup launcher here. Pick it up and go up the stairs. Around the corner at the top are 3 enemies - aim at the middle guy and fire, and you should kill at least 2 and get the trophy!