As of a patch issued 28th May 2014, this once glitched AND incorrectly described achievement can now be obtained once the cumulative score of 2,500,000 points has been reached.
It's a self explanatory achievement, but here are some tips for reaching 2.5 million as painlessly as possible.
1). Use the multipliers. Look at the locations of the 2x Word or 3x word multiplier tiles and play words that make use of them. The higher value the word, the better. I will call this first big word on the board the "base word".
2). Make combos. This is where the big points are. Build new words off of your base word, even if they are only 2 or 3 letter words. Trust me, you will get used to playing words like AI, EN, OI, YO, EM, AE, AY, etc. Make sure you find as many combos as you can, even if that means frantically swapping letters around on a whim. You do not get punished for playing words that don't exist. The developers call this behaviour "Goldmining". Take a look at the video to see what I mean.
3). Importantly, use up all of your letters in a single combo as often as possible as it scores extra points. If your base word just won't allow you to make these full combos, do not waste too much time on it.
4). Once you have found all the combos you can for your base word, change the base word. Often you can alter it only slightly, for example, change "TAMED" to "MATED", and then play almost all the same combos again to double your score. These small changes will save a lot of time, so find every variation you can. If you can pluralize your base word, do. If your base word is "GAME" see if you can make "GAMES" and don't forget "GAMED", "GAMER" and "GAMERS". Make as many combos as you can for each, and be quick about it.
5). Perform those steps as quickly as you can, and once you have exhausted all the base words you can see in that place on the board, if there is still time on the clock, switch to another place that preferably has a multiplier tile, and start goldmining all over again.
I rarely score below 3000, and often score over 10000. With the above tips, an average player should be able to average around 4000 points per game. At that rate, it would take 625 games to achieve the cumulative total of 2.5million. Each game takes about 3m 15s (2m 30s play time, 45s wait for the next game), so after around 34 hours play I'll be at the required total score.
See the attached video (by the developers) if you need to visualize these tips in action. (Thanks Hybridchld for the heads up on that!)
Even if you aren't very good at it, it doesn't really matter, it'll just take longer. This game is so addictive it'll come in time without you even thinking about it anyway.