Achievement Description:You must blow up 10 vehicles in 10 seconds.
Go to a local ammunition supplier and obtain some explosives, the rocket launcher and standard grenades work the best. Once you have these, choose anywhere in town that makes you happy, preferably somewhere busy. Go into the road, steal a car, and park it so that it blocks traffic. Then steal another one and make the pile of cars bigger. Once you have at least 10 cars in a big pile, start detonating them, and then any surrounding cars until the achievement unlocks.

Here is the achievement in action by way of a slightly different method: