When you have a character reach Tiny Tina do all of her side missions until you make it to the final stage of her tea party. Here she wants you to lure Flesh-Stick to her little cove to join her tea party. Since you can fail this mission by killing him you can actually repeat that over n over for experience. On top of that if a second controller is available and you wish to make another character but don't want to level them up the normal way you can have that created person join your session and have them run all the way to Tiny Tina. If you keep them in front of her and your main character in front of where Flesh-Stick walks out you can kill him and re-accept the mission as fast as your trigger finger allows making anyone reach roughly 20-21 in a matter of minutes.