QUAKE 4

Once you reach 250 kills with the gauntlet you will unlock this achievement. This needs to be completed in ranked matches so just do the following.

1.Start up the game
2.Press "A" to go to the main menu
3.Select Multiplayer
4.Select Xbox Live
5.Select Custom Match
6.Select Ranked Match
7.Apply these settings
Game Type:Death Match
Game Length:Endurance(Endurance allows you to get 30 kills in each match, followed by Average with 15 and then Quickie with 5 kills)
Match Type:Ranked
8.Find some friends that have this game and would not mind boosting with you, you can press the "Y" button to go to your friends list and invite them to your game, Or while looking at this game on this website near the top of the page under "Genre" it will say "X amount tracked gamers have this game" in parentheses it will say however many gamers want to boost just click on that and it will take you to the page listing all of the gamers that want to boost, contact one or more of them saying you would like to boost in this game.
9.Once in a game with how ever many people you need to get 250 kills with the gauntlet, you start the game with the gauntlet (its the chainsaw on your hand), just meet the other player/s somewhere and kill them, once you reach your 250 kill you will get this achievement when the game ends.

Just to add to what Nick said, make sure that you get it as close to 250 as possible. Don't go to far over the mark as it sometimes resets your progress. This happened to me and I had to do it from the beginning.