I got to 18 by myself but stopped because I had a friend on her way and she doesn't like to play COD. Seemed really easy though and I probably could have gotten to 25 or 30 without too much difficulty.
http://youtube.com/user/LeonidasRules - My youtube channel with all of my FFX vids
got to 29 on co op then 24 25 on solo i think when theres 3 juggernauts theres a pretty good chance i die but basically i just run around and buy a ton of claymores c4 plant then and wait for them to walk over it. seems like holding out in a building gets you killed at higher lvls.