Here is an extremely easy way to get this achievement.
At the start of the game set the "Turn Time" to "Off", and the "Match Time" to 30 minutes. Now make sure that you go first (keep restarting until it happens). Finally, just leave the game running (have your controller plugged in to prevent it disconnecting) for most of the 30 minutes, then enter your word about 10 seconds before the countdown ends. The AI will not have time to enter their words.
You can also do this on the "Child's Play" and "Floats Like a Butterfly" achievements. The achievements are not stackable, so you will need to do them separately.