The Sky's The Limit trophy in Borderlands 2 is story-related and can't be missed.
You will unlock the trophy once you complete the 10th story mission "Rising Action". The steps below (taken from the Xbox 360 guide on TA by mast3r 0r0m1s):
*** Spoiler - click to reveal ***- Go to Lt. Davis
- Remove Power Core
- Install Power Core
- Go to center of town
- Cycle the ignition primers
- Find Roland and pick up Eridium nuggets
- Go back to center of town and give to Firehawk/Lilith
- Head toward the Fridge
- Fight your way to the door