This can be attained after leaving the planet Pandora and boarding the ship Sanctuary.
To gain access to Moxxi's Bar you will need to complete a few tasks once you get on board after which speaking to Ellie allows you into the bar.
Once there simply go to the slot machine (Loot Boxer) on the left of the entrance and press X to spin the reels. There is some element of luck with this achievement but here it was accomplished in 4 spins, costing $247 each.