It's not just K/D that levels you up, although it is a major factor. If you play online with other people then try to be that player that always revives, opens doors etc, basically be a team player as much as possible. If your like me and play solo, just get to high rounds consistently, don't go down too often. Get to about 25-30+ a few times and you should be good. If you have Mob of the Dead then play that map constantly, it's the easiest map in Zombies history and I constantly get to 65+, and my highest round on any other map is 24.