Grand Theft Auto: San Andreas – The Definitive Edition

Before starting Sweets mission Los Sepulcros (roughly 25 missions into the game), where you have to kill a gang member named Kane, get a 2 star wanted level. It will not disappear without going to pay and spray. Progress with the mission to where you kill Kane. At the cemetery, cops will spawn, but not attack you. After taking out all the Ballas + Kane, DO NOT enter the car with Sweet. Instead, start killing the random cops that spawn. They drop pistol ammo, so you will not run out. A LOT of cop kills later(40+-), you'll eventually get the 6 star, then you can enter the car with Sweet, and "try" to get back to Grove Street. Good luck because there are tanks that will blow up your car in 1 hit. If you die, you'll restart at the cemetery with your original 2 stars. Just redo the mission, avoiding the cops, and race home on 2/3 stars.

After completing black project, you can reenter the military base through a breakable fence in the south/south-east corner near the hangar, and get 5 stars.

Drive to and enter the rhino, then leave the compound again and blast every FBI car coming.
For me, i needed to drive away from the base as driving in circle arround never got me a 6th star. I suggest you get back on the road, fire at FBI cars before they're too close and work your way back to your airfield to store the tank if you want to do the 12 levels of Vigilente.

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This is very easy to do in the Hunter. The downside is you have to buy the airstrip and finish Flight School first, but it is unlockable quite a bit sooner than the Rhino.

Take the Hunter from the Abandoned Airstrip, fly over Area 69 to get five stars and then head east toward LV. Rain missiles on the residential area right next to the highway, both on pedestrians and cars. Ignore the Hydra and the other helicopters, but keep circling to avoid missiles and to keep your angle. It should only take 10-20 seconds of missile mania to get the last star.

Bleeps and bloops everyone!

This is not as hard, simply once you have completed Black Project head to the military base, steal the tank, drive out the base and keep driving away further from it, destroying everything on your way. Took me around 3 minutes of driving and I reached the 6th star. Achievement will unlock upon reaching the 6th star. Here's also a video guide.

Once you unlock Las Venturas, go to the Four Dragons Casino and get a 3 star wanted level before going inside. While inside you are protected from the swarms of cops but there will be security guards trying to kill you and respawn infinitely but slowly. If you reach the back of the casino, there are spots where a guard will always spawn at the same location when you look away so you can take him out to grind the wanted level. This is also a good method to grind your weapon skills without the shooting range.

Once you finish the 100% achievement, a Rhino tank will always spawn under the bridge on Grove Street. It’s not invincible but it can take a ton of damage. Use this and it’s cannon to easily cause enough havoc to earn yourself 6 stars.