Simply put, this is unlocked by beating a full game of Horde Lite. Horde Lite is the 25 wave horde game-mode. This can be completed in about 60min, 45min if you've got a good group of people on it. There's a boss fight every 5 rounds rather than every 10ish as it is on regular horde mode.
I did this using the soldier class with mainly a lancer and enforcer. Skill cards I would prefer for this would be: Assault Rifle Capacity, Assault Rifle Damage, Assault Rifle Magazine, Hammer of Dawn Strike, Cover Boost, and then the Execution Bonus.
I'd recommend being in a party with everyone whenever you play horde mode... being in a silent group is kind of painful for your mental stability in horde as a whole.