There are five loot midgets that you must locate and kill in order to obtain this achievement. They are as follows:
Mini Steve: you can find him anywhere, but he has the highest likelihood of spawning in the red chest near a Claptrap in Locdown Palace
Tuxican Wrestler: located in the first lockers in Lockdown Palace; it is possible for another to spawn later in the palace
Meat Popsicle: located in a fridge in Lockdown Palace past the vending machines in the mid section of the level; a level 61 Meat Popsicle can be located in a hidden room under T-Bone Junction
Crimson Shorty: located in a Crimson Lance chest at the end of Road's End
Dumpster Diver: located in a dumpster (of all places) in Motorhead's Garage and can only be reached when the "Little People, Big Experiments" quest has allowed you to access this room