When you go to Multiplayer, you have to choose 'Random Player' instead of one of the friends on your friends list. Win 50 times.
You have to keep all 50 wins in your results history -- apparently if you remove your wins from the list, then the game no longer counts them. [Thanks to Mtld for figuring that out.]
This is boostable! After you play against someone paired up via Random Player, you can choose the finished game from the list and select 'Rematch'. The rematch also counts as a 'random player' match. If you and your opponent collude, you can keep playing each other as often as you like.
This probably won't work for long, but if you have access to two computers and can play at an odd time, you can get matched up against 'yourself'. I was able to do this in only 3 games at 4am Monday Pacific time. (As more people play, this will become harder and harder.)