The Pistole is a high-powered pistol that holds two shots in the clip, and is often a one-hit kill on many enemies.
The best time to get this trophy is the couple of Chapters when you are on the moving freight train, which is about halfway through the game. There are Pistole lying around everywhere on the train, and many of the enemies will drop them when killed, as well. There are far more than enough enemies on the train, and plenty of ammo, for you to get the 20 kills needed fairly quickly.
Note that if needed, you can restart checkpoints while on the train and the kills will still count toward your total.
The trophy will unlock as soon as you get the 20th kill.