Halo 3

You must do this in the Lone Wolves matchmaking playlist. Try and get matched in either High Ground or Isolation.

Get a Mongoose and drive at full speed directly into an opponent. It may take a second attempt and a bit of luck since the Mongoose is only a lightweight vehicle.

Another tip is to try and run down a person while their shields are low as this will heighten your chances of successfully mowing them down.

Get a group of 4 or more people together, make them all preffered players. Then go into your system settings and have everyone change thier language to Portugese. Start a party chat and go into matchmaking. Change matchmaking search preference to use "Language." Having preferred everyone you will all be matched up, if everyone changed thier language. This works for all of the Lone Wolves online achievements. Just take turns

took a few tries but I did it on Snowbound during a BR slayer match. Basically I took the Mongoose and drove it around the outer edge of the map within the trenched areas as to avoid being too exposed and fortunately I was able to catch a guy running at the right time. And I believe he had full shields too as he appeared to not be in combat. Here's a clip you can download to Halo 3 to see how I did it.

http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3...

For this achievement you must go into a Ranked FFA (Free For All) lobby and look for people that are in combat and wait for they're shields to drop and hurry up and ram them as fast as you possibly can with the mongoose. If you see them near some kind of solid object ram them into that so that you can increase your chances of getting the achievement. For example on High Grounds there are tons of rocks out in the open for you to ram them into and there are 2 mongooses(mongeese). That doubles your chance of getting a mongoose to ram them with. Hope that this guide helped you and the others that read this. Happy Achievement Hunting!

i only want to add this if you are planning to boost the free for all achievements make sure that you have players with the same lone wolf rank skill for example between 20-30
and that you prefer each other so that you can find each other easier
happy huntings :)

In addition to boosting with the language trick, try having at least 1 person using the original Halo 3 disc (not the ODST mulitplayer disc), without having any DLC downloaded. This narrows down the list of maps you will get, and offers more maps with the Mongoose. We tried this for an hour and kept getting newer maps without the Mongoose. 1 person (without any DLC maps) switched to the Halo 3 disc, and we got a Mongoose map immediately.
To get this, just take turns running each other over with the Mongoose. It helps to have to have someone shoot up the player getting run-over, to strip his shields. This will make the roadkill much easier to obtain. To get on the Mongoose, hold RB. To drive it, just push LT forward. This will be more difficult to obtain on some maps (such as Isolation, due to its landscape) but is possible using this method. Just find a flat portion of the map to use, and give yourself enough of a runway, to gain enough speed.
BTW, not all versions of Snowbound have a Mongoose. It can also be a Ghost.
Some of the original Halo 3 maps with the Mongoose:
High Ground
Valhalla
Last Resort
Isolation
If you do have the Halo 3 disc and the DLC, you can always delete the DLC and re-download them again afterwards, at no cost.
If anyone has any more suggestions, comment and I'll update this solution.
Here's how to do the language trick (credit preppypoof):
http://www.xbox360achievements.org/forum/showthread.php?t=17...

This is the hardest achievement in the game, in my opinion. This can only be done in a Ranked Free-For-All match. I suggest Lone Wolves. Now, you have to use a Mongoose. No questions about it.

Find a Mongoose on the map. I suggest Snowbound or Isolation. You need to gain a certain speed to kill somebody with the Mongoose. Try to get a good distance and then, go for it.

You also damage the person that you are trying to run over so that you have a higher chance of killing them.

And keep a look out for enemies. You don't want somebody to kill you right before you run the person over. (Trust me, it's happened to me)

Isolation in the Lone Wolf playlist is the key. Drive around in a circle along the outside of the map. You will eventually come across someone in your path. just hit them.

Will work better if they are damaged or in battle (that's how i did it)