To unlock this achievement you need to be ranked in the top 50% of the people who played that game on the scores menu at the end of a game.
You can get this pretty easily by just playing normal games, you do not have to guess loads to unlock it.
If you are struggling you could use a boggle solver to help you on your way.
http://www.danvk.org/boggle.phpThe above solver is a pretty nice one to use.
The achievement will unlock in the scores menu at the end of the game if you qualified.