Difficulty: Any
Mission: Prison Break
After the CCTV sequence, and after rescuing Alejandro you'll be met by riot shield guys in a hallway as you release the other prisoners. Pickup one of their riot shields. The next area will be where we can get this achievement accomplished.
Once you get to the next area in question, you'll drop down and immediately sprint to the double-doors in the back left corner. Just when you're about halfway I'd advise tossing a flashbang and then start spamming
to begin your beatdowns. It took me a few tries on Veteran, but ultimately I was able to kill three guys on a few difficult lives. I cannot say for certain if they have to be consecutive or done on the same life, but if you fail you can always just revert to the previous checkpoint and try again.