This is a glitch where you can easily earn as much XP as you desire very quickly. Just follow these steps.

1) In Tundra Express, complete all of the optional quests for Tiny Tina until the reach the quest, "You Are Cordially Invited: RSVP" quest.

2) You are supposed to lure Flesh Stick into her cave but if you just kill him, you will be awarded around 2,000 XP every time.

3) When you kill him just go back to Tiny Tina and restart the quest and continuously kill him.

4) A faster way to do this glitch is with a friend. Have one person stand by Tina while the other player stands by Flesh Stick's spawn, and he should keep appearing.

Here is a video tutorial in case there is any confusion: