I know a lot of you play Horde and if like me this Onyx medal seems to be getting hard to get here is a way to boost it:
You need the 'friendly fire' mutator for this to work!
Start up a horde game on Insane with the Friendly Fire mutator activated. Guest sign in the other player or do it with a friend. Select the map Checkout because the command posts are closer then any other map. The person wanting the Founder ribbons goes ahead and buys the command post. Both characters change to there smoke grenades, and stick each other with them (Press B next to each other). *IMPORTANT - THIS DOES NOT WORK OTHERWISE* Select 'Restart from beginning' after you died NOT restart from current wave.
This will work in splitscreen and you don't lose points on your overall horde medal even though it will keep giving you -points everytime you do it. You can do this about 2.5 times in 1 minute.