You need 2 controllers to do this.

On split screen, make a Search and Destroy game. Go to options and set unlimited lives, unlimited score, and the rest doesn't matter too much.

Start the match, choose the guns you want to level up. Then choose the team and class for the person you don't want to level up first, so that they will be defending the targets. Then on the profile you actually want to level up your guns and your overall level, choose the other team and class.

Then grab your bomb and go to the other player and shoot him in the head. He will always spawn in the same area so just stand there and keep shooting them in the head so that you get more points. I spent only about a hour doing this and i got from below level 20 to about level 40.

This only works offline!