EDIT: The servers are shut down as of February 8th, 2011. This is now UNOBTAINABLE.
Getting this done now is a must for completion. Please read the thread in the link below. It would seem the thread in the link below has changed. Now the guy with the top 50 account appears to be trading for help with other achievements from what I can tell. Thanks for the comment pointing this out.
This is a link to an x360a thread how I got the achievement. It requires you to ladder a dummy account up to 3,500 points. These points then help to maintain the top 50 account which gets you the win. This is the most realistic way of getting this achievement now, assuming the account is still top 50 (it was in the 40's I think when I got the win). Hope this helps.
http://www.xbox360achievements.org/forum/showthread.php?t=17...