To easily complete the "Store Wars" mission, enter a police car and log onto the computer. Search for an arson crime. Do the Vigilante mission, and after killing criminals, pick up their flame throwers. Start the "Store Wars" mission and quickly park two cars at each end to make it difficulty for the vans to get past. Let the vans drive towards you, equip the flamethrower, and hold A for two seconds to easily make the vans explode.