In the Tundra Express, complete all the optional quests for Tiny Tina that lead up to "You Are Cordially Invited: RSVP". This quest requires you to lure Flesh-Stick into Tiny Tina's cave so that she can put him in a trap. The trick is to just kill him. Depending on what level you are (Level appropriate on first playthrough is anywhere from 1800-3000 exp per kill, playthrough 2 is anywhere from 2000-4500 per kill). After you kill Flesh-Stick, simply go back to Tiny Tina and restart the quest. If you are online and playing with friends, just have 1 friend stand by Tina while someone else stays by Flesh-Stick's spawn, and he will pop up instantly each time.