This achievement is called "Rhino Squad" and it is achieved by getting 10,000 frags (kills)
The FASTEST way to get this achievement is by doing a ARENA CTF match using any of the "X" on 1 method. For example, I recently got 6 players together and we had 5 people in a quake "party" (go into the lobby BEFORE you search for a game and invite all your friends to it making a "party" in which all of you in the "party" will be on the same team regardless) and than have the one person that wants the kills (the sixth person) search on there own to find the game. When the game starts it will be 5 on 1 and have the one person get the "doubler" power up and run to the other teams base where everyone will be spawning and have the five people just running at ya giving you kills. I know this method works. We did a Average match (20 minutes) and we were getting around 450 - 600 kills per match. So the more people that you can get to help you boost the better. (on the CTF matches if no one has captured a flag in the time limit it will go into a "sudden death" and will continue to let you rack up kills until someone returns a flag.
Side note: You don't have to do it on Average it can also be done on Endurance or Quickie.
2nd Side Note: In my experience this is also a nice way to rack in the "Impressive's" and "Excellent's" the only draw back I have seen is that as exactly 255 the awards reset, so watch out!
UPDATE: I just got done doing this method 5v1 using the doubler and i ended up with 870 kills in and endurance arena CTF. (my excellents did reset at exactly 255 tho...thank god i already had that achievement)