Instructions
Turn on your game console. Load up MW2 and invite your friend in to a party with you.
Create a private match and select "Cage match" as the type of game you want to play.
Invite your friend in to the game lobby. Click on the "Create a class" tab and change your six different classes so that each class has different weapons and perks within them. This will allow you to switch classes during the game and pick up experience for each different weapon and perk.
Access the options for the game and change the total kills needed and the total time to maximum. This will give you the longest possible time to collect as many experience points as you can.
Begin the game. Kill your friend a number of times with various weapons. This will give you substantial experience. Once you have as many points as you would like, have your friend quit out in the middle of the game to the dashboard. This will result in him receiving no deaths from the match. Set up the match again and this time have your friend kill you numerous times so that he can get experience. Repeat the process until you have as many experience points as you would like.