Mongoose Mowdown

Mongoose Mowdown is considered one of the more difficult Halo 3 multi-player Achievements to get, along with Two for One. It is awarded for splattering an enemy with a Mongoose in a ranked free-for-all playlist. This achievement usually unlocks the Scout Armor shoulder piece for Spartans, but the shoulders have been unlocked without this achievement for some players. The achievement awards the player 5 Gamerpoints. So far, the only ranked free-for-all playlist is Lone Wolves.

It is represented by an orange circle with a Spartan driving a Mongoose, three red stars, and a flying body.

Tactics

 * A basic tactic for this achievement is to find a Mongoose in an open area, like on Sandtrap (only on Legendary brawl) or High Ground, and put the Mongoose at high speed at another player.
 * One of the easiest ways to achieve this is to play on Isolation, and drive a Mongoose into the Shotgun spawn point. People taking cover there are sitting ducks. If you are lucky, some nice players will notice that you are trying to get the achievement and let you drive right into them. A secondary way is to drive at two players dueling and splatter the winner of the duel.
 * A slightly more risky way of getting this Achievement is to play on High Ground and keep a close watch on the Ghosts. When someone gets in a Ghost, follow it with the Mongoose in case they are hijacked, then you can run down the temporarily stunned (and likely shieldless) opponent.
 * Another way on High Ground is to just take a Mongoose, (which are always in ample supply), and circle the base, out onto the beach, circle back and drive through the tunnels (which ever one you choose), and back into the base. Chances are, is that you'll get someone by surprise, and get the achievement. The trick is to never stop. This lowers the chances of you being killed, and just make enough of a course change so that your always pointed towards your target.
 * Another tactic is to have someone on the rear lower the targets shields and then the driver to splatter them.
 * The reason that this achievement is so hard is because only a head on hit with a Mongoose will splatter a shielded opponent. A great way to get this achievement is playing Crazy King on Isolation or Boundless, as all the players will be grouped up and will likely have low shields.
 * It is important to note that the Mongoose is prone to explosives, so sometimes your splatter could be due to explosives propelling the vehicle to one spot, or your route of splattering someone could be stopped. Fragmentation Grenades and Brute Shots are the ones that are especially annoying. Rocket Launchers usually destroy the Mongoose within a shot, depending how close it is.
 * Remember that the Mongoose is a prime target for a Spartan Laser, mainly on High Ground. It is easy for someone to backpedal and take you out before you can make contact. Thus, using the element of surprise and charging at someone caught up in a fight is a useful tactic.
 * A good strategy of doing this is by going on any level with a mongoose while players are fighting and run down someone when they're distracted by the other players, especially if they are standing still in a relatively open space.

Links

 * Halo 3 Achievements
 * Mongoose