I thought this was an impossible achievement but after checking the tinternet I found out how to get it. You need to download your own clip!
Surprised nobody else has posted the solution here.
First you need to create a second Xbox Live profile on your 360 if you dont already have one. Silver will do and it has to be at least 13 years old to sign in to the EA server. Make sure the new gamertag is on your friends list.
Next you need to upload a BO clip. I made mine about 2 seconds long but I'm not sure if the length of the clip makes any difference to the download time.
Now its time to start downloading. Sign in with your new GT and go to "Burnout clips" then "friends clips". Select your tag and then "view clip". As soon as its finished downloading 100% hit your guide button and sign out, then sign back in again. You will go back to the main menu and you get signed out of the EA server. Now you repeat the process. Go and view your clip again and the times downloaded starts to go up.
When you have downloaded it enough times to get into the top 20, sign in with your usual tag and go view the top 20 to recieve your achievement and get 65 gamer points.
You do not need to go back to the dashboard, or switch the 360 off.
You do not need to save the clip, only view it.
You do not need to watch the clip after you download it.
You do need to download it 100%.
You do need to sign out and back in again after each download.
I read that EA reset the top 20 daily and somewhere else said monthly, so you might want to check it before you start so you dont have to spend too much time doing this. Be warned, it is very boring. Most of the time is spent staring at loading screens! If your lucky 20 or 30 times will do it. I done 80 the first night and my girlfriend done 100 the following night while I was at work. I couldn't be bothered waiting for the reset.