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I know a lot of people have trouble farming him, and I haven't seen my way anywhere online so I thought I'd post it here, as gamefaqs message board posts often turn up on google.
To farm Tumba, go to the Wildlife Exploitation Preserve, and go to where the creature slaughterdome is. If you don't have the creature slaughter dome, it is the end of the map opposite the entrance, I believe the first spot with ammo/health vending machines. If you have the creature slaughterdome, enter and exit, then go out the door to your left, towards the New-U station at the wildlife exploitation preserve. Tumba will be in that first outdoor area right as you are leaving the building, along with the skags and stalkers. He appears roughly 50% of the time, you'll easily be able to tell whether or not he is there by the sight of a skag barfing fireballs towards you. Once you're done killing him, or if you don't see him at all, save and quit. Reload the game and you should have spawned by the creature slaughter dome/ammo dump health machine. Lather, Rinse, Repeat.
I got a head (not sure if normal drop from him or not) and a Deliverance in ~1 hour