Here's how to unlock this trophy eezy-peezy:
Go to Chapter Select and pick Chapter 4 - The Dig, and load it up on Very Easy difficulty. Progress a very short ways into the level until you get to the shed where you get the grenade tutorial, next to the boxes of grenades. After the cutscene, skip through the tutorial and run out toward the guys coming at you. Make sure you have your pistol equipped. Run up to each enemy and shoot him ONCE FROM THE HIP and then punch him with "square". A single punch will kill him. About 20 to 25 enemies will spawn here, so go to town.
Occasionally, you will get a guy that does not go down, though. Don't worry, just reload the checkpoint and try it again. Took me about 5 or six tries to get it, but not too hard to do.
While you are doing this, you should be working on the following trophies as well, so see my other solutions for them while you're at it:Note: The trophy will unlock as soon as you get the 10 kills in a row in this manner.