Ok, for a while I was a little confused about how to even play horde in POP. There are a couple of ways but for both you need to be in a clan.
1) When opening a reward box you will sometimes get an item that looks a little like a hammer, this is a ‘Thumper’. This is essentially an item you can use to start horde mode.
Swipe across to horde and click the Battle button. This will then prompt you to select a thumper. You then have to wait for a clan mate to accept your invite and away you go. You do not have to complete the all waves to get credit towards the achievement.
2) The other way to start is to wait for someone else to use their thumper. A prompt will appear on your screen saying that someone is looking for a horde partner. Click on the prompt and you will join that player.
You need to play with 20 different players to get the achievement to pop and I would encourage you to try and finish as much as each game as you can as thumpers are quite rare and you need to complete 500 waves of horde for another achievement.
You can check progress by hitting the “missions” button (bottom right), click the achievement tab and take a look at “Never go alone”
It may be worth recording who you play with or alternatively just accept every invite you can until you get it to pop