After completing the "A Dam Fine Rescue" mission, you will find Sir Hammerlock inside Sanctuary offering a quest called "Mighty Morphin'", which involves taking an injector and injecting Varkid Cocoons with a special serum. Injecting the serum into a cocoon produces a Badass Varkid. Killing one yields around 1,900 XP during your first playthrough, depending on your level. As long as you do not complete or turn-in the quest, you can kill all the enemies, choose to "Save and Quit", and reload the game to have the enemies respawn. Repeat this as many times as desired.