***Servers for this game were closed down April 13th, 2012 but have been temporarily reinstated. It is unclear how long this will last so quickly finish the online achievements like this one now while you have another chance!***
Original solution - To get this achievement as fast as possible, follow these steps:
1. Create a 1 or 2 second clip (the shorter it is, the less time it takes to download) and share it.
2. Make a fake Silver or Gold account and add it to your real account's friends list.
3. Sign in to your fake account and view your friend's clips.
4. When the clip is finished downloading but before it actually starts playing, go to the Xbox guide and sign out. You should now be booted back to the Burnout starting screen.
5. Simply sign back in with your second account and repeat until your real account's clip is in the top 20.
6. After you're there, sign in with your real account and go to the top 20 clips. *You should now unlock your achievement!
And remember, the leaderboards for the top 20 reset every month on the 25th. If you're going to try and get this achievement, you'll save a lot of time doing it right after they reset.