Mongoose Mowdown
From Halopedia, the Halo wiki
Mongoose Mowdown is considered one of the more difficult of Halo 3's multiplayer achievements to get. It is awarded for splattering an enemy with a Mongoose in a ranked Free For All playlist. This achievement also unlocks the Scout Armor shoulder piece for Spartans. The achievement awards the player 5 Gamerpoints.
It is represented by a orange circle with a Spartan driving a Mongoose, three red stars, and a flying body.
The easiest way 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.
Another 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 which lowers the chances of you being killed, and just make enough of a course change so that your always pointed towards your target.
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, as all the players will be grouped up and will likely have low shields.
Sometimes, if you are playing on Isolation, you can wait near the Flood tunnel until someone comes out, then go through because odds are someone is following them trying to finish off their kill.
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