This achievement is glitched in a positive way. While the exact conditions are unknown, you may be awarded several cleared Horde waves without actually completing them. Some gamers have even reported being credited Horde wave completions without even playing Horde first!
As far as a solution, there
are two methods
is really only one method to go about clearing the required number of Horde waves; a co-op method
and a solo method.Co-op
For the co-op method, simply join an active Crew, where "active" means many different people are dropping Thumpers on a regular basis for you to join. If your current Crew becomes less active, or is not as active as you would like, simply "Crew shop" by joining different Crews in search of active members.
You can also use the forums here on TA to look for active Crews, or create your own Crew and recruit members, though this will cost you 1000 silver coins.
An update has caused this method to no longer be viable due to the game not granting new players enough coins to quickly reach level 2, thus being allowed to participate in Horde.
Thanks to Rascalov USA
for first pointing this out to me.
Original solution below:
If you have any questions or suggestions, please let me know in the comments and I will update the solution appropriately. If something in the game is updated and renders portions of this guide obsolete, then again, please comment or message me so I can update the solution.