You must complete the mission Bright Lights, Flying City to unlock this achievement.
*** Spoiler - click to reveal ***
Bright Lights, Flying City is the 11th mission of the Borderlands 2 main story line.
Mission List in Order:
My First Gun
Blindsided
Cleaning Up the Berg
Best Minion Ever
The Road to Sanctuary
Plan B
Hunting the Firehawk
A Dam Fine Rescue
A Train to Catch
Rising Action
Bright Lights, Flying City
Wildlife Preservation
The Once and Future Slab
The Man Who Would Be Jack
Where Angels Fear to Tread
Where Angels Fear to Tread Part 2
Toil and Trouble
Data Mining
The Talon of God
Ending