Now, as you may have noticed, killing 'Jimmy Jenkins' is one of the Miscellaneous challenges required for the 'Challenge Accepted' trophy for Borderlands 2. Where can you find him, you ask? Unfortunately, he is a VERY rare chest spawn, mostly from ammo crates and cardboard boxes.
However, this is a near-guaranteed way to spawn Jenkins within a few runs only, since it ALWAYS spawns 4 loot loaders from 4 cardboard boxes each time when under the RIGHT conditions as below (including TIME condition!):
1) The host of the game must have the Doctor's Orders quest active, and at 0/4 (meaning no quest items picked up yet for that quest); it becomes available from Tannis in Sanctuary upon reaching Wildlife Preservation.
2) To make runs to the 4 cardboard boxes faster, the host should also have the Creature Slaughter DLC installed.
3) Simply run from the start of Wildlife Preservation to the Creature Slaughter Dome (aka Natural Selection Annex), if you have the DLC; enter and exit to set the Creature Slaughter Dome as your new reload point, or if you do not have the DLC, you will have to make do with the further start point of Wildlife Preservation.
4) Run from the Creature Slaughter Dome until you get past the Super Badass Loader guarding the holding cells. In one of those rooms, there is a big chest, some ammo crates... and FOUR cardboard boxes.
5) Gather your whole party, if more than one player, into the room, and WAIT for at least 1 min or more (this allows the area to finish loading the random 4 loot loaders into the cardboard boxes). If you open the boxes too soon, there will be no loot loaders and no chance to spawn Jimmy Jenkins.
6) Do not ever pick up the quest item in one of those 4 boxes (for the Doctor's Orders quest), not EVER; otherwise it will make this not work anymore. So simply have the host back up his/her save before doing this first, just in case.
7) When done right, a loot loader always pops out of each of the 4 boxes, giving you four chances to spawn Jimmy Jenkins out of 5 different types of loot loaders. If no Jenkins yet, reload game.
8) Repeat Steps 3 to 7, until every player in your party has killed a Jenkin each.
That is all, and good luck! =3 This worked for me and my two friends; 4 loot loaders from 4 cardboard chests each single time. The most important thing to keep in mind is the time condition, as mentioned in Step 5 above!