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^ This is where I did it. I believe it's chapter 1, checkpoint 3 (the checkpoint that begins with you leaving an elevator).
Just kill everyone in that room with w/e weapon you have, then pick up the shotgun from the last guy you kill around the corner before the cutscene of another guy busting through the door. When you get to the parking garage, take out the shotgun and you can kill 5 guys (4 guys in the section of the parking garage you're in, and then one more guy you can shoot through the gate). Once you kill the guys, reload the checkpoint and you start in the parking garage again, rinse, repeat. Just make sure you don't hit the gate switch on the wall to load the next checkpoint and you can just keep doing it until you get your platinum.