Start puzzles and enter words until you have 500 total. You do NOT need to finish puzzles. Just find some easy words and move on. There are a total of 120 puzzles in the game and getting the other achievements should have you already at 70-80 words, so you only need to average 4-5 words. To make it even easier/faster, you can use an anagram solver like wordsolver.net.
Full video walkthrough: