This was the first REAL trophy I earned (aside from the 'You Go To Hell' trophy). It can be tough when you just start out, but having played it on the Xbox, I had some practice. What you need to do is use your Boner (
) and simply aim for the head. I got it in Garcia's apartment, as there are many weak baddies. Let them get a few feet away from you, and watch the laser that comes from the gun. It may take some time, but with enough patience (and learning the enemies movements), this is actually an easy trophy to unlock.
I also found a easy spot to get this,I'm not 100% on the chapter (I believe it to be after Bughunt). Once you've escaped the town, you'll be in a swampy moaty area. As you progress, you'll come to an area with cages, Garcia will say, "Who put these hellmonkeys in cages?" You'll need to use your LightShot on a cage to free one of them to douse the goat head to destroy a switch.
DON'T RELEASE IT. Instead, position yourself in front of the cage and with some maneuvering, you can kill the demon in the cage and it'll continue to respawn. This will allow you to get it with no effort.