Getting this achievement should be very easy - complete 100 full rounds and the achievement should unlock.
Since you don't even have to be playing, you could even just keep the game running when you have completed all other achievements and you're done - that is, if you're actually playing only for the easy achievements. Even if you're actually playing this shouldn't take long.
has been letting the game run on his phone and apparently games in which you don't do anything don't count anymore. I can't verify this since my game can't connect to my xbox account anymore but I think it's safe to say that you now have to (try to) find at least one word for the game to count towards your total. Still, 100 games is not incredibly high so if you have finished all other achievements and if you're just going for the 100% then I advise to play a few words now and again while you're doing something else, like watching tv or pretending to work.
However, this achievement seems to be kind of buggy, it doesn't unlock (right away) for some people. What I did was play a few more games and it still didn't unlock. Then I closed the game and started it up again, got the achievement after the first next full round completed (extra information: I played this on my android Phone, didn't use an emulator and I haven't played any other Wordament version). This seems to have worked for some other guys too, others just kept on playing until it finally unlocked.
Unfortunately, as for now there's not much else to go on. Just keep playing if it doesn't unlock, try closing the game and starting it again, see if it unlocks after a new full round completed. Good luck!
has determined that the 'complete ten games' needs to be done on one single language (and therefor this one too); the completed games don't stack between languages. So keep in mind when going for this achievement, that you will need to have at least 100 completed games within one language before the achievement will unlock.