Another storyline achievement so this is not missable.
Following the completion of:
This is the penultimate storyline achievement.
For spoiler reasons I won't list what happens, but you'll have to head to head to a very familiar place (if you played Borderlands 1) and retrieve some information on the location of the final boss fight. There is one very large and very dangerous enemy on the way, but it is actually optional and not a required boss fight.
The next (and final) storyline achievement is...