Instructions
Start the multiplayer matchmaking service. Play in a ranked free-for-all game to gain this achievement. Choose the "Lone Wolves" playlist to make sure the game type is correct.
Play a map that has wide-open space and a mongoose -- an ultra-light all-terrain vehicle -- available. Look for maps such as "Isolation", "High Ground" or "Boundless." These maps have ample space to get up to speed and maintain it while looking for prey.
Board the mongoose as soon as the match starts. You get the "Mongoose Mowdown" achievement by running an opposing player down with the mongoose.
Get the mongoose up to speed. You need to hit opponents with enough force to kill them. Even running a player down from behind at top speed may not be enough, so always keep your speed up. Avoid getting trapped in corners or areas where you will need to stop or slow down.
Keep driving in laps until you spot an enemy. Avoid heading directly at an enemy. You make a very large target for rockets and the Spartan laser, and the mongoose is small so it is easy to dodge. Look for targets that are heading away from you or, ideally, that you can attack from the side. Surprise is your chief weapon.
Aim to strike the enemy directly with the center of the vehicle. Only a small area in the front of the mongoose will result in a direct hit. Blows struck with any part of the vehicle other than the front center will result in a glancing blow, knocking the enemy away but not killing him.
Exit the match after you have run down an enemy and gained the achievement. The scout shoulders will now be available for use in the armor customization screen.