This has to be done on Horde, difficulty set to Hardcore. Mutators help, waves 10, 20, 30, 40 or 50. Everyone must play as the Onyx Guard from the DLC. This is easiest in Private so you can conrol the map and the difficulty.
The important thing to note is that you MUST survive the wave. If you die, other players will get the achievement, but you won't.