The An Old Flame trophy in Borderlands 2 is story-related and can't be missed.
You will unlock the trophy once you complete the steps below (taken from the Xbox 360 guide on TA by mast3r 0r0m1s):
*** Spoiler - click to reveal ***- Go to Frostburn Canyon
- Fight way through cave
- Refer to this solution to get Goliath, Meet David Achievement
- This is basically a fighting your way through the cave mission.
- After you get through all the checkpoints you'll be at the lair.
- Find out its an old friend and have to protect them from waves of enemies.
- After you retrieve the second nugget for them talk to Firehawk and mission complete.