ok.. grab a mate over xbox live or plug in a second controller to make it faster.. If you aint got those two factors, search online for matches.
To get this, you need to pass 3 waves without failing, then it will present you with a wave challenge, these can vary from killing x enimies to completing the wave in a time limit to no players to die in that round.
This is clearly visible in the top left corner of ur screen. If you play horde alot, then this will be no problem for you. You should get this pretty fast, really easy onyx medal..
Doesnt need to be done in one session.