Easiest way to get this is in the final chapter Conversion Tower. While playing through the level you will get to an area with a structure in the middle with un-lit 'bombs,' one will be on the left side of the structure.
Once you enter one of the Impalers will come in with a few other Chimera. Wait until the Impaler gets far enough in and shoot the glowing 'bomb' on the right side on the floor - It will kill it instantly. Then, let the rest of the Chimera kill you and repeat until the trophy pops.
If you do not abuse this, there will not be enough of the Impalers or Executioners in normal game play to get the trophy, as your progress will be at 66.2% (+/-). If more are needed, once you have completed the game use : Load Level --> Conversion Tower --> Impalers and it will place you outside of the door for the above room.