There is a banner above the game when there is a digram. It shows how many digrams there are in the puzzle. Focus only on using the words that use that digram tile. With a little luck and brain power you should be able to get the achievement. Worse case scenario, use the puzzle solver below. Enter all of the information and read the list. Now you have the answers. After the achievement unlocks, I suggest backing out so it won't count on the leaderboards.
Wordament Solver -Use at your own risk and cautiously. You can be banned from the game if caught using it.