Just play the game as you normally would. If you don't destroy enough after you've finished, just do the event "Rolling Thunder" where you are in a Thermobolic Tank that shoots mini-nukes. Finish playing that event, and then just drive around in the Tank, destroying clusters of cars with the cannon. If you run out of ammo, just replay the event. I did this as an alternative to just running over cars, as it seemed that crushing them didn't always count as destroying them.