You can just use the players from splitscreen (like player 2,3 and 4) to kill and get xp.
Use this code:
bind DPAD_DOWN "xblive_privatematch 0 ; xblive_rankedmatch 1 ; onlinegame 1 ; fast_restart"
(change the bind if you want)
What this code does: it makes the game think that you are in an online match. So you can go to splitscreen (or private match) and use this code and gain xp for online.
Basically you just bypatch and stuff like you normally do, but instead of going online go to splitscreen. Wait there 10 seconds then click on online (so your online data is downloaded).
Now go to splitscreen and you will see all your kills/deaths/ks/hs etc. will be added to barracks (Remember to go to online after splitscreen so it saves your changes).
Once you join a normal game and finish it (or leave it) everything gets added to the leaderboards.
SO you can rank up in splitscreen, become level 55, go to online and prestige and go back to splitscreen and repeat process. Enjoy.