Here is the full list of all 35 Final Fantasy XIII achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
Felled a heavyweight of the lowerworld wilds.
Held every weapon and accessory.
Completed all Cie'th Stone missions.
Earned a 5-star rating for all Cie'th Stone missions.
Triumphed over undying lowerworld souls in seven fierce battles.
Discovered buried treasure with a little help from a chocobo.
Completed all high-level Cie'th Stone missions.
Completed all mid-level Cie'th Stone missions.
Toppled a green terror and cut an oversized succulent down to size.
Dealt 100,000+ damage with a single attack.
Passed Titan's trials.
Took over 10,000 steps on the lowerworld surface.
Fully developed all characters.
Mastered the Medic role.
Mastered the Synergist role.
Mastered the Saboteur role.
Mastered the Sentinel role.
Mastered the Ravager role.
Mastered the Commando role.
Witnessed the dawn of a new crystal legend.
Earned a 5-star rating in the battle to determine the world's fate.
Defied destiny's charge and embarked on a different path.
Completed all low-level Cie'th Stone missions.
Traveled to the world below, seeking a way to alter fate.
Discerned the full attributes of 100 enemies.
Recognized the true threat to the world's future.
Took the fight to the enemy's door.
Carried the burden of guilt to the end of the line.
Made plans to infiltrate enemy-occupied territory.
Evaded pursuers, though memories of the past still gave chase.
Resolved to be more than a victim of circumstance.
Slipped through the net and lived to fight another day.
Strode into danger's den and paid the consequences.
Survived the Purge to confront a greater peril.
Took the first steps toward challenging an unjust fate.