I got this on the Casino level (third one) after a few attempts. You start in the bottom left corner so use the shortcut to appear at the top of the screen then make your way down to the car and interact with it by pressing Y. The car will turn on and mow down the guests by the entrance which should get the achievement to pop. It may take a few tries due to there not being 15 people (I got 13 and 14 a few times) but keep trying and you'll get it. Video below if you would like to see me doing it.