You must be on Xbox Live for this cheat.

Have a friend host the lobby and select any new character (level 1) to play as.

Have the host count down until he starts the game, and press the Select Character menu at the same time as the host starts the game.

Once you've spawned, the character menu should be up.

Select your highest level character and your current character will now be whatever level your highest character is!

This is useful for the first play through because your attacks are incredibly strong. Enemies don't stand a chance!