This can be an annoying trophy, but there is a real easy way to unlock. It's also best to do this after you have completed the game, as it's not extremely necessary and would be extremely hard to do throughout the game.
Load up Chapter 2, and set the difficulty to 'Very Easy.' Progress through the chapter, and you can either start this when you reach the two guys on top of the truck, or when you meet up with Chase. If you start with the the guys on the truck, then you can get this done in Chapter 2. If not, then you can unlock at the beginning of Chapter 3. You cannot restart a checkpoint or die; this must be done in one life. Restarting a checkpoint resets the counter.
Shoot a bad guy (I shot without aiming), and follow up with a punch by pressing square. You will either punch him in the face, drop kick him, or punch him in the gut-area. It must be done from the front, and not a stealth kill from the back. It only took one bullet and a punch. After ten in a row, you are done. I unlocked it at the beginning of Chapter 3.
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