After you complete the Tina part of the story mission (getting her missiles) , a new side quest should show up for her tea party (Cordially Invited, or something), and once you need to lure Flesh Stick you can just kill him, failing the mission but still giving lots of XP for his death. Restart, Kill, rinse and repeat. Should net about 4000 XP at first, then get progressively smaller. Other than that, the Bandit slaughterdome and Lynchwood are good places to get some good XP. Killing Spiderant queens in the Dust will also give some decent XP.
Okay: farm a level off Wilhelm and open the chest each time for bonus loot. Then fight those ninja turtles for a level or even two. With Flinter included, it takes like an hour or so. Also bunker is good for a few levels as well as just killing everything in lynchwood while doing those missions plus the Hyperion contract one. That'll get you pretty damn close.