I know this has been posted, but this is my take on it.
Have 5 people join up in a private lobby. Preferably people with the same goal in mind. It will make it so much easier if you have Infinite Ammo and the Super Reload mutators enabled. Have it set to Hardcore difficulty. You can start it from wave 10 if you wish since having the mutators makes it insanely easy and you will not have to worry about money. Once the host selects the map, make sure your character is the Onyx Guard. After all 5 players have the Onyx Guard as their character, the host can start the game. From there, all you have to do is beat the wave.
It only seems that if you live through the round you will unlock the achievement. With the group I did it with, 3 of them were dead and did not unlock the achievement when I did.
Below is a video to go with. We failed the first time through, the lambent bezerker did work on us, video shows the lobby and picks up from the end of our first try to the round were we won.