This one is pretty self-explanatory, as all you have to do is win 50 waves in Horde mode. It sounds pretty time consuming, but it's not too bad. I was able to pick it up on my second day of playing the game.
But first, you have to unlock Horde mode. To be able to play Horde mode you have to be at least level 2 and be the member of a Crew. Then to play a round of Horde you can either use a Thumper from your inventory. or wait and be invited by someone in your Crew.
Games I've seen have 2-3 waves each, so this should take, at most, 25 games of Horde unless you have trouble clearing a wave. But if you have a good enough Squad this should be pretty easy. Just be aware than some waves have a boss at the end with special moves which can make the final waves more difficult, especially if you go up against RAAM.
NOTE: According to Oaawara2 and NutriWhip it can pop before the conditions for this achievement are met when logging in.
NOTE: Memo popped this achievement without playing a single game of Horde mode and without being in a Crew, so it seems as though progress can be gained regardless of whether or not you play Horde at all.