First, create a traffic jam in an enclosed area, such as the Booth tunnel, by blocking off both lanes with two cars. After the cars back up, steal some of them and squeeze them between the others so that there are three or four next to each other. Then, throw a grenade at the cars in front and switch to your rocket launcher to destroy the others.
Park ten vehicles at any gas station around the pumps, then get an RPG and aim it at the gas pump. All ten cars should explode with only two shots.
Note: An RPG or grenades are required for this trick. Simply go to the Westdyke neighborhood on the third island ( found towards the top of the island). A bike shop is there that has eight bikes in it. Take a bike in with you and stack them all in the corner, then shoot a window and shoot an RPG at the car outside. Then, just blow up the bikes. You will probably get a one star wanted level for stealing bikes, then probably one or two more stars for the explosions.
To easily unlock the "Chain Reaction" achievement, have the "Remove wanted level" code activated in your phone. Go to the "AutoEroticar" store in Alderney to find a group of cars in the garage and in the lot. Stuff all the cars into the garage. As police see you breaking windows and moving cars, they will try to arrest you. Enable the "Remove wanted level" code to make them go away, then continue to stuff the cars in the garage. Once you have ten or more, throw some grenades in the garage and to unlock the achievement. Make sure you have completed the other achievements such as "Walked free" before attempting this, because you may not be able to earn it once the code has been enabled.