This has to be done in 'Private' Horde unless Epic do other 'Hardcore weekend's (where the default Difficulty in public Horde was set to Hardcore)
Its as simple as getting 5 of you together, (or 3 people and 2 of them with Guests) starting up a game (where you each select 'Onyx Guard' as your skin) and beating the first boss (or any boss, go for 50 if you like ^^) just make sure everyone is alive, anybody dead wont get the Achievement :)
Mutators (Infinite Ammo, Super Reload, Insta-Gib) if you have them will make your life easier, you can even try starting at 10 if you feel you can handle it without the cash from the previous 9 waves.
the Host will also earn 'The host with the most' Achievement if you play on a Horde Command Map (Blood Drive, Rustlung and Azura)