Not every round you play will have a theme word in it. The theme word will appear at the top of your screen, with a star next to it. Basically find any word related to it and the achievement pops.
Using the word solver is recommended.
Image provided by StayonTarget.
WARNING: Use the Wordament Solver at your own risk. The developers haunt the leaderboards for anything suspicious. They will ban you if they suspect you have cheated and you will then no longer be able to get the achievements.
Wordament Solver - http://dev.360surface.net/Wordament/EnterBoard.aspx