Used Car Salesman
From Halopedia, the Halo wiki
Used Car Salesman is a Halo 3 Achievement. It is awarded for destroying a vehicle with three enemies in it in either a ranked online playlist or in campaign. Unlocking this Achievement gives the player the Scout Helmet for Spartan Armor customization, it also awards the player five Gamerpoints. It is represented by an eight-pointed yellow star on top of an eight-pointed red star. On top of this are three white stars.
Walkthrough
Method 1
The easiest way to get this achievement is on The Covenant, after you disable the Third Tower and are on your way to the Citadel (load "Rally Point Bravo"). Get in the Scorpion tank, and destroy the Prowler that comes around the corner. Alternatively, you can steal rockets from the Marine on the Mongoose riding along-side the Scorpion tank and weaken the Prowler with your Assault Rifle before destroying it with rockets.
Method 2
The next way to get this achievement is by playing on The Ark. Get to the part with the crashed Pelican, and there will be a Rocket Launcher there. Once you have killed off the Brutes, two Prowlers will come your way, hopefully one of them will have three Brutes on it. Do not fire the Rocket Launcher straight away as it will only kill the Brutes but not the Prowler itself and will not count towards the achievement. Try shooting it with a smaller fire arm then finish it with the Rocket Launcher, that way the vehicle will be destroyed.
Note: Go after the second Prowler not the first, the first one is indestructible. One suggestion is throwing a Frag Grenade, then shooting it with the Rocket Launcher.
An easier way is in the hanger of Crow's Nest. Two Phantoms come inside the hanger, three if playing on Heroic and Legendary. There are two turrets in the area, or you can run up and stick the turret with a Plasma Grenade (Spike Grenades aren't strong enough to blow the turret to pieces). You can also wait until after Forward Unto Dawn drops off the three Scorpions and the Gauss Warthog. There are multiple opportunities where a Phantom is stationary and in range. Using the Scorpion's primary weapon, roughly 5-7 direct hits will shoot it down. Another simple tactic, would be to use a Spartan laser against a Prowler, of course with a well aimed shot.
Method 3
On the campaign level The Covenant, save your Spartan Laser energy until you get to the Wraith guarding the tower. Take a Ghost and drive up the naturally formed ramp to get to the platform in front of the Shade Turret. Kill the Wraith as soon as possible! Also, a few Ghosts with Grunts driving them roam the area so wipe them out with the Warthog. Eventually two Prowlers come out and both have at least three Brutes in them and you should be able to wipe it out with a well placed Spartan Laser shot. There are more Prowlers in the Assault on the Citadel part later in the level so if you fail at the first tower, use the Scorpion to knock out the Prowlers there.
Method 4
The actual easiest way to get this achievement is on the level Crow's Nest, when the Phantoms in the hangar drop off Covenant infantry, you can use the Machine gun Turret to destroy the Phantom's Plasma Turret. Once the turret explodes, you should get the Used Car Salesman achievement, even if there are no Brutes in the drop ship. If the Turret explodes and you don't have it then keep blasting the belly of the ship and you should get it. If you still don't get it, try blowing up the turret, and throwing a grenade up through the gravity lift. Sometimes if there are enemies still inside the Phantom, throwing the grenade alone may get the achievement, though this is rare. This is a glitch due to the game most likely mistaking the destruction of the Phantom's plasma turret for the Phantom itself being destroyed. This method only works on the second drop off of the Covenant. Or easier, using the same phantom throw a plasma grenade onto the turret at the front. It will stick and, depending on where it landed, destroy the Plasma Turret. It only takes a few melee hits to knock off the turret on the phantom.
Method 5
You could also load three Marines into a Warthog, then kill other marines, making them turn on you, counting as enemies, and destroy it.
The easiest way to do that is on the level the Storm on Normal go to the part where you are fighting the first anti-air Wraith, you have to pick up the missile pod (in the little bunker where Marines are fighting the Covenant) kill the Covenant and kill the Marines with the missile pod and the Warthog should be sitting across from the bunker entrance it will start shooting at you to prevent being killed go below and shoot the missile pod out the door and kill the warthog and get the Achievement.
Method 6
On the campaign level The Ark, get a Plasma Pistol or a Fuel Rod Cannon at the start of the level. In the part where you meet up with the Marines who are going to the crashed Pelican, you will find a Rocket Launcher near the Pelican. Swap your other weapon for the rockets and quickly switch to your Plasma Pistol/Fuel Rod. There will be a full Prowler approaching to the left of the Pelican. Charge your plasma pistol/ shoot your Fuel Rod once and switch to your rocket and finish the helpless Brutes off.
Method 7
Play Co-op on the first level, kill the Arbiter to get Plasma Grenades, then after you kill the Brute Captain Major in the big tree (at the start), a Phantom will appear to drop more troops; throw the Plasma Grenades at the Phantom's Plasma Turret to get the achievement, be sure that the Arbiter does not shoot at the turret, if he shoots you won't get the achievement.
Method 8
Play the level Tsavo Highway until you get to the point where you save a bunch of marines at a base on a steep hill. Kill all the enemies and inside the building on the top of the hill there should be a machine gun. Wait until the Phantom arrives to drop of more brutes and destroy its turret. You should get the award from that.
Method 9
Another good way is on the campaign mission Sierra 117, destroying the turrets on any of the phantoms will count, getting you the achievement.
Note: Another method involves simply using the Spartan Laser while in Matchmaking to destroy a Warthog with three passengers, common on Capture the Flag variants.
Method 10
On the level The Ark, start at the part where 343 Guilty Spark stings the marine with his laser and continue until you reach the cartographer and the cutscene appears. Whether you watch or skip the cutscene, it matters not. After the cutscene a phantom appears, throw either a plasma grenade or a spike grenade at the front turret. Even though the phantom doesn't get destroyed, through a glitch in the game, if the front turret is destroyed, you get the achievement.
Multi-player
The easiest way is to get it on a large level with Warthogs/Scorpions/Prowlers. Like Standoff. Grab the Rocket Launcher or the Spartan Laser. Wait for the other team to grab a Warthog and destroy it. It is best to do it on a CTF gametype, since their players tend to use team vehicles.
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