1) Complete 100 full games (loading screen to loading screen).
2) Achieve 2,500,000 points.
3) Place in the top 250 of any of the "top" leaderboards (top players today, weekly, monthly, all time).
#1 and #3 have to be done on the same language. So, if you completed 100 games in English, you can't use Italian for the leaderboard part (unless you do another 100 games in Italian).
Also, if you are in the top 250 of any of the leaderboards before you hit 100 full games, and you then hit 100 full games while in the top 250, you will have to drop out of the 250 and re-enter it.
Plus, keep in mind that the leaderboards are not updated in real time. It may take up to an hour for your scores to count and your position to update... you can scroll all the way to the bottom to see when the leaderboard was last updated.
Finally, this achievement seems to be glitchy in that it may not pop when it's supposed to. You may have to completely exit the game and reload it, and it may pop when it gets past the "connecting to Wordament" part. This is not a guarantee though, it may simply not pop at all.
If you are positive that you have met the requirements, and you are aware of everything that I put here, exit the game and reload until the achievement finally decides to pop. Personally, I had to reload about 50 times over the course of three days before it decided to pop. I have heard of people having no problems, and I have heard of people who can't get it to unlock at all... this is simply what worked for me.
I ranked in the "top weekly" leaderboard in addition to the "top daily" one. Not sure if it matters, but that's how much I had to screw around with it to get this to unlock.
Also note that if you pop this on one version of the game (WP / Win 8 / Android), it will unlock in the others even if you have fell out of the leaderboard. I would assume iOS is included in that list, but I haven't played that version and can't confirm.