This is one of the more time consuming trophies. You will need to complete the "Get Some Answers" mission (story related) as well as completing all missions in the Rust Commons, which in truth for the trophy, covers everywhere in the game in regards to places you will have to go to. However, aside from the main story line, a few missions and the claptrap rescues, you can find these missions in the following locations:
-New Haven (Scooter, Helena, Marcus, Bounty Board)
-Crazy Earl's Scrapyard (Earl)
-East Rust Commons (Middle of Nowhere Outpost - Bounty Board)
-Claptrap Rescues (New Haven, Tetanus Warren, Scrapyard, Krom's Canyon, Old Haven, Trash Coast)
Additionally, the 2 most common missions missed are Green Thumb (Middle of Nowhere) and Bandit Treasure (Old Haven). Below is a list of all missions required to unlock this trophy (in no particular order):
Claptrap Rescue: New Haven
Claptrap Rescue: Tetanus Warren
Corrosive Crystal Harvest
King Tossing
Is T.K. O.K.?
Like A Moth To Flame
Power To The People
Up To Our Ears
Scooter's Used Car Parts
Hidden Journal: Rust Commons West
Scavenger: Submachine Gun
Seek Out Tannis
Meet 'Crazy' Earl
Claptrap Rescue: Scrapyard
Get Off My Lawn!
Today's Lesson: High Explosives
Hair Of The Dog
Claptrap Rescue: Krom's Canyon
Earl Needs Food...Badly
Missing Persons
Two Wrongs Make A Right
Middle Of Nowhere No More: Investigate
Middle Of Nowhere No More: Fuses? Really?
Middle Of Nowhere No More: Small Favor
Middle Of Nowhere No More: Scoot On Back
Scavenger: Shotgun
Altar Ego: Burning Heresy
Circle Of Slaughter: Meat and Greet
Circle Of Slaughter: Round 1
Circle Of Slaughter: Round 2
Circle Of Slaughter: Final Round
Hidden Journal: Rust Commons East
The Next Piece
Altar Ego: The New Religion
Altar Ego: Godless Monsters
A Bug Problem
Relight The Beacons
Green Thumb*Smoke Signals: Investigate Old Haven
Smoke Signals: Shut Them Down
Claptrap Rescue: Old Haven
Jaynistown: Spread The Word
Jaynistown: Getting What's Coming To You
I've Got A Sinking Feeling...
Wanted: Fresh Fish
Jaynistown: Unintended Consequences
Jaynistown: Cleaning Up Your Mess
Bait And Switch
Claptrap Rescue: Trash Coast
Earl's Best Friend
House Hunting
Another Piece Of The Puzzle
Not Without My Claptrap
Firepower: All Sales Are Final
Firepower: Market Correction
Firepower: Plight Of The Middle Man
Jack's Other Eye
Bandit Treasure: Three Corpses, Three Keys*
Bandit Treasure: X Marks the Spot **Here are video guides of Green Thumb and Bandit Treasure missions, courtesy of WikiGameGuides:
Happy Hunting :)