Not really legitly :/ most you can really do is become a better player by killing more zombies and going down less. a lot of people (not all) with higher zombie ranks just sit in glitches all day killing zombies and hardly going down. I don't have a problem with glitching but I have a problem with the ranking system -_-

Just play an easy map like mob of the dead and use the Hell's Redeemer or play Buried and jsut use the paralyzer and sit on the roof you have to fly to and don't open the chair and sit there killing them. Easy to get kills and try not to down. It won't take one game though. I finally got shotguns having like 78k kills. And you will need loads of kills so i'm talking 60+ rounds per game and play a couple.