This achievement will unlock once you have an average word score of 20 points or more across all online games played, not just a single game after you have played your first 10 words.
If you've played the WP version a lot you may find you are quite a bit below the required 20 points average. I was on 14 average with around 110 games played and it took about an hour of only scoring 40+ points per word to reach 20 points average.
This can be checked in the statistics menu and is called 'Avg Move:'
Credit to Minioger for this info:
passing and swapping count as a move and decrease your average as they are 0 points!!
I ended up getting it by starting a game and getting a 20+ point word or resigning right away! Resigning doesn't hurt your average score (it doesn't count as a move).
EDIT: Credit to Humpurton for letting me know local games DO NOT count.