While in the Tundra Express, complete all the optional quests for Tiny Tina that lead up to "You Are Cordially Invited: RSVP". This is a quest in which you need to lure Flesh - Stick into Tiny Tina's cave so that she can put him in a trap. The trick with this 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). Once you kill Flesh - Stick, just 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 every time.