Here is the full list of all 10 Wordament achievements worth 50 gamerscore. It takes around 1-2 hours to unlock all of the achievements on iOS.
Complete 10 rounds
Rotate the board and find more words in a round
Beat all your Frenemies in a round
Play Wordament in more than one language
Find a digram word in a round
Find a theme word in a round
Complete 100 rounds
Place better than 50% of the players in a round
Find the longest possible word in a round
Find words that use all 16 tiles at least once in one round