You need to find 2400 words with 4 Letters.
Use following setting:
Bonus Tile: On
Time Attack: On
Word Find: On
Banned Letter: Off
This setting will show you all available words in alphabetical order for the current round, and will also help finding words for the other Achievements (Short & Sweet; High Five!; Master of the Sixes; Lucky Number Seven; Octorithy)
You can track your progress with the Achievement-App.
It is still a boring grind, but much faster than finding to words yourself