This can be a tricky one. Sometimes there are words that are easy to spot and you can get the achievement. Other times, it isn't so obvious and the longest word is a very uncommon word you likely have never heard before. You can test your luck while going for other achievements or use the wordament solver below. Put all the letters in the language you are playing on website. Solve puzzle. Now find the longest word on the list and use it. I suggest after you get it to just back out and not count the game to lower risk of being caught.
Wordament Solver -Use at your own risk and cautiously. You can be banned from the game if caught using it.