Start up a game of Unlimited and just use the word unscrambler and enter the Bingo word first. This is the word with the most letters denoted by the gold stars. This allows you to skip straight to the next round. Just repeat for each round. Once you have twenty rounds complete the achievement will pop but continue for another ten rounds as this is a requirement for the 'Grand Master' achievement.
This achievement can also be obtained in the other games modes such as lighting where you just need to solve the twenty words in a row (i.e. not going to the next round).