Ok this is a simple guide.
To do this in co-op, you must select 'Multiplayer' after choosing the game mode. This will allow others to join your game. Have one player start a Horde game and advance until there are only a couple of enemies left in Wave 3 - Round 15. Either invite your co-op partner or have them join and once the last enemies are defeated, the achievement should unlock for the person who just joined, provided you didn't get hit and took any damage. Repeat this for the other person(s) in your group.