Just got this achievement. This has to be done in original Scrabble Flash, not the Scrabble Flash 5 Word.
In Scrabble Flash the point of the game is to spell words to get to 25 points before the other player, each letter is worth a point, so 5 letter words are worth 5, 4 are worth 4, 3 are worth 3.
Know this right away, once you score 25 points, the game is over and you win.
Simply start a game with 2 controllers. If you need to go to a word generator online for the 5 letters. You will want to save a 4 or 5 letter word for your last word. Get up to exactly 24 points with your player. With the 2nd player just spell 3 letter words. Once you have 24 points, spell a 4 or 5 letter word and you get the win and the achievement.