My brother and I made it to 21 on a splitscreen. We went online afterward to see the legit highest level, and found all sorts of bs. I hope Treyarch resets the leader boards soon.
We used alot of teamwork and launched a coordinated attack on each wave. It's funny, because when I play 4 players on Live we have never made it past 16. If we had two more players that could understand military tactics and not "lone wolfing" it then we could probably go well into the 30s.