This cheat requires 2 controllers.

Sign both controllers in and head to splitscreen MP. Set the game type to Search and Destroy and set the match rules to 1 life bomb, time between 5 to 15 seconds, the rest doesn't really matter. I recommend using rust but its up to you.

After you start the match run and plant the bomb. After it's planted quickly run to second player and kill them - go for headshots for the most points.

When the bomb detonates you should recieve 10,000 exp and can level up quickly.