As with GTA games prior to San Andreas, you can participate with side missions that involve vehicles. One such example is the Vigilante mission, which allows you to take a law enforcement vehicle and pursue criminals. To start it, get in one of the appropriate vehicles and press the right analog stick in. Doing this will start a timer and spawn a criminal randomly in the area you’re residing. You will have to kill the criminals you chase, which can also draw the ire of the police themselves, to come after you. If you get out of the vehicle or if your current one is destroyed, you have 60 seconds to find a new police vehicle to continue.
Things will get more difficult as you reach higher levels, as there will be more criminals and they will begin carrying more powerful weapons. You will need to complete all 12 levels in a single sitting, though, in order to get this trophy/achievement. Your reward for doing this is a 50% increase to your maximum armor, which helps out a lot during missions. The only problem is that the better vehicles for it don’t appear until later. If you wait until completing Flying School with all Gold on the tests, then you can have a Hunter spawn at Verdant Meadows. Note that either that or the Leviathan can spawn in that spot, so if you want the Hunter, come in from the west.