So I have been completely bored of zombies lately so I wanted to try something different. So play solo, do everything normally i.e. buy revive and open doors. But don't buy a gun until you get to the mystery box. Once at the mystery box, try for one of two items (in the end you want both) the Ballistic Knife and the Thunder Gun. Once you get the Ballistic Knife go to the lander near Speed Cola and buy the Sickle.

Once you have both guns, Juggernaut, Revive and whatever other things you want like Claymores and other perks; Use the Ballistic Knife/Sickle combo only and only bust out the Thunder gun when there are a crazy amount of zombies. Make sure you link the landers and launch the rocket so you can pack-a-punch both weapons.

This technique, while seemingly uneffective, got me to round 27. And if you want to try this as a team of people (of course with different secondary weapons) hit me up. My GT is Aptuhtude.

Happy Knifing!