If you go to
http://wineverygame.com/boggle/helper.pl fill in the box with the tiles on your game and then just go down the list of words and find the longest word there and make that word on your game, the achievement should be yours. I did find a few words that came up did not work on the game, so you may need to try a few.
The achievement will pop as soon as your get the longest word not when the round ends, so if you think you have done the longest word and the achievement does not pop then try a different word.. thanks to DJ Drty Chapstk help.