Depends on how far you are in the game - if you have the other islands opened up, go to the army base near your airstrip and steal the tank. It's not easy, but once you're inside the tank - they can't stop you. Drive it home, park it in your garage and save the game. Now get back in the tank, hit R3 to toggle Vigilante missions on and you are ready to go. You drive around and crash into or shoot the bad guys for increasingly high amounts of cash. At level 12 you get 150% armor - I got to level 36 in just one go round and made over $700,000. The last kill alone was worth $50k. The only way you can be stopped is if a cop opens the tank and pulls you out, otherwise, you are invincible. Enjoy!