At the end of Tiny Tina's Tea Party side quest you're required to lure a psycho named "Flesh Stick" back to her home. If you shoot and kill Flesh Stick rather than luring him back you'll reap a huge XP boost (750xp/kill on level 18 and around 1,800xp on level 17) which can be repeated by simply talking to Tiny Tina again and restarting the mission.
This exploit works best during co-op play. where one person stations where Flesh Stick spawns and another at Tiny Tina. Doing so will enable you to repeat this process in quick succession until the game will no longer spawn Flesh Stick again. At this point you can run back to the start of the area, exit and return, or (faster), save & quit then reload the game. Doing so will allow Flesh Stick to respawn again.