Needed: Sniper, Bombs or Rocket Launcher, Wanted Levl Cheat and acess to the upper part of the island on the left.
Go to the top of a building in the left island. You can get up via helicopter or stairs. Destroy stuff with weapons(dont use the sniper yet). Get 6 stars. When you get 6 stars wait for a tank. It should be tackling the building by now. Use the Wanted level cheat to reduce stars to zero. Jump off the building, steal a ride and chase the tank. Block it(dont touch or you'll blow up) with the ride and then get off. Sniper time. Use it to kll the soldiers inside. You have to do this cause the doors are locked and you can open them when you kill the people inside. Hurry and bring the tank to your garage to repair it(Tanks aren't accepted in Pay n Spray). Enjoy your tank.