There are 4 leaderboards. Fruit, Arcade, Zen and Classic. The Fruit leaderboard doesn't count as I tried this one.
You have to have another tag higher than you then you beat him.
I actually earned some high scores and then added new friends and I already rank too high on them.
So keep that in mind.
The achievement unlocks after the game has ended.
EDIT: The Solution below about the Silver tag method is a brilliant idea. I suggest making it on Xbox.com to avoid wasting a free gold your console will give you if you haven't done so already.