Where is Jimmy Jenkins?
We found him in one of the structures around the Pipeline in the Arid Nexus; I'll try and find the location's name tonight. He was in a random Hyperion ammo/cash chest on top of the building.
It appears he spawns randomly; I found him in a coop game once, then searching the area again solo I failed to find him. Sidenote, in coop he only counts for the single player that deal the killing blow. Share with your buddies or farm him solo; if you seriously want to max out the challenge it'll take a long time.
I've also found him in one of the Loot Midget boxes in the Wildlife Exploitation Preserve (there's 4 of them near the place where you first try to find bloodwing, with the shielded pens). I think he might spawn anywhere a Hyperion Loot Midget (LWT Loader, Loot Gun Loader Midget, Loot Jet Loader Midget) can spawn.
If you have a character that is doing the "Wildlife Preservation" and the "Doctor's Orders" missions you will always have something jump out of the boxes near bloodwings first holding cell. Eventually Jimmy Jenkins will bo one of those loot midgets. There is no where else where you can have loot midgets every time so this place works the best.
When I finally killed Jimmy he jumped out of the first and fouth boxes so I was able to kill him twice in this room. Having the above mentioned missions active is required.
According to random message boards on the internet, he is located at or around Crater Lake.
I just found him in a cardboard box in T.K.'s house. Made me want my own little Jimmy Jenkins to hide in boxes.....
He has a chance of spawning where "trap boxes" are. So wherever you come across a loot midget or anything that come out of something: REMEMBER IT! Eventually Jimmy might come out. Anything after the wildlife reservation has the best chances!