As the achievement states, you only need to reach wave 26. You must complete wave 25 and then your achievement will unlock upon wave 26 starting. How to Set-up the Lobby:
You can use a private custom lobby for this achievement or public custom matches for this achievement. Just remember that it always needs be hoard with 100% damage (default) and change the time to 15 minutes.Before Attempting:
Through trial and error, I've found that doing this with only two people is easiest. You cannot do this achievement solo and doing it with 3 or 4 people adds more enemies and makes the whole thing harder. You shouldn't use bots at all because the bots are terrible. You should also play with someone who is familiar with the game. Both players should be using characters that have quick and large area signature moves. The best characters I've found for hoard are Queen Nai, Mirage, or even Hattori. Queen Nai is best however because of here neutral signature on daggers is perfect for hoard. You should also use the dexterity stance so you can get moves out faster.In-Game:
You and the other player should cover both portals. Kill all the enemies as fast as you can. If you don't have any enemies on your side, maybe help your teammate out. On boss waves, one person should be focusing on the boss and the other should be killing all the additional enemies. However, the quicker you kill that boss, the less adds you have to kill.
It really is straight forward. I've heard of ways to cheese the boss enemy and quick kills on him, but never found a strategy that consistently works. It's all a matter of trying to remember as many of the waves as possible and how to beat them taking minimal hits to your portal.
If a particular wave is giving you trouble, refer to this video: https://www.twitch.tv/videos/409124953
It features one of the top pro players and a caster for the esports.