During the first level "Bellwood", use Humungousaur to smash every car you come across. Once you've destroyed 15 cars, the achievement will unlock. If you've it to the end of the level (where you climb to the roof to fight the boss), you can destroy the two cars that are on the street before you climb up; the first one is just before you climb up, and the other one is further down the street. Destroy these, then start climbing the pipe, and jump into the fire to the right of the third pipe (Just before you've reached the top). When you spawn, keep smashing the cars until the achievement pops.