This one isn't really a cheat but it helps. First of all, make sure you have a friend that wants to do this.

Second, search for 3rd Person cage match and when it's searching for a match invite your friend that will help you boost.
Then simply Kill him repeatedly with whatever gun you need kills or headshots for.

You can also complete challenges to unlock titles and emblems. Make sure your friend respawns with Tactical insertion and get him to place the insertion close to you so he can respawn and you can keep on killing him.

Trade of doing that and you will get to Level 70 in no time. I went up 10 ranks a day by doing this.