Pretty easy to get. Bellwood is the first level, and there are enough cars to pop the achievement. Simply switch to Humongosaur and bash the vehicle a couple of times to wreck it. Should be enough cars to get the achievement before the end of the level (I lost count and had to resort to the method below).
If you get to the final boss of the level without smashing 15 cars, just die and you'll respawn on the street right below the pipe you must climb to get back to the boss fight. There is one car very close at hand and another right down the street. Smash them, and if you still don't have the achievement, then climb the pipe with Spider-Monkey and jump to the right into the flames on the nearby roof. Respawn and repeat as needed.