There's three (3) types of final rounds: Acro Lash, Comic Lash, Word Lash. It's random which one you'll get, but it seems that Word Lash is far less common. It took about 25-30 games for it to even appear for me, but it's in there. Just keep doing rounds.
The achievement will pop immediately when "Word Lash" appears on the screen, you don't have to finish out the final round.