This is the hardest non-leaderboard achievement in the game. You must find every themed word. These only happen every 4 or 5 rounds. A themed game is when there is a banner above the puzzle and it will say the theme. This is anywhere from animal sounds to outer space. A lot of them are easy once you get one. Like an example is when a word can be used 3 times. Hiss, Hissing, Hisses. This is usually not the case. Normally the themes focus on less common words and if you fail, you have to wait a handful of rounds before you can attempt another theme. The best way is to go to TrueAchievements.com and search for the game. Find the specific achievement here and go to the solutions. Many people have contributed to a list with words they have found for that theme. The problem is a ton of those only have a couple of them and you will have wasted a lot of time using Control F to search. The way I was able to do it was with the Wordament Solver and I quickly scanned the solution table. It was still a pain, so good luck on this one.