The Farewell, Old Girl trophy in Borderlands 2 is story-related and can't be missed.
You will unlock the trophy once you complete the story mission "Wildlife Preservation".
*** Spoiler - click to reveal ***- Go to Wildlife Exploitation Preserve
- Find Mordecai
- Infiltrate shipping yard
- Find holding cells
- Find Bloodwing
- Rescue Bloodwing
- Weaken Bloodwing
- Get Claptrap upgrade
- Collect Slag samples: 0/10
- Deliver upgrade to Claptrap