Instead of frustrating yourself by trying to shoot single enemies from cover, wait on this trophy until you get to Chapter 19 - Unwelcome Guests. As you begin the chapter, you are in a large room with gazillions of MP-40 machine guns lying around. Grab a bunch of them and get ready for a huge onslaught of ghouls...or zombies...or whatever the hell those things are.
Shoot them using only the R1 button. Do not use L1 to aim. In fact, this is actually the easiest way to kill them. When the creatures stop coming, don't leave the room. Just Restart Checkpoint, and have at it again until you get to 20 and unlock your trophy.
NOTE: Save your game after getting the medal, or it may not save your medal points.