I had previously done this on the Xbox 360 version.
On the One I imported my level 69 Soldier and on *Playthrough 1* progressed through the General Knoxx DLC. During the "Prison Break: Try Not To Get Shanked" mission, I opened a crate with a mini Steve in it, killed him and the achievement popped.
As such, if you've done it on the 360, looks like you only need to find one of the loot midgets if you've imported your character over.
The midget locations (taken from IncrediBoy's solution from the Xbox 360 version of the game - Solution for Sneaky Little Buggers in Borderlands
1. Mini-Steve - Inside a red chest in the southeast bandit camp in the Deep Fathoms area.
2. Meat Popsicle - In the Sunken Sea area, get on the freeway and stop underneath the Dahl sign. On your right you will see a set of stairs, walk down and find a fridge around the corner of the small building with Meat Popsicle inside. -- Edit: REDWINGS05 reports that this is incorrect, "It's not in the sunken Sea area. He is in the very first main area."
3. Truxican Wrestler - Immediately upon entering Lockdown Palace, search all of the lockers in the first long hallway.
4. Dumpster Diver - Accept the "Little People, Big Experiments" mission from Thirsty and head to the waypoint marker. Take down Motorhead and search the dumpster within the garage.
5. Crimson Shorty - Just before you enter The Crimson Armory, there will be two Crimson Lance chests. One of them will contain your final Loot Midget.
You may also find the other solutions for the the Xbox 360 guides for this achievement useful: