Here is the full list of all 28 FINAL FANTASY II achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Defeated 300 monsters.
The rebellion commenced against the Palamecian Empire.
Defeated 100 monsters.
Earned all achievements.
The empire destroyed numerous towns with the Dreadnought.
Blew up the Dreadnought.
Obtained a ship from the pirate Leila.
Defeated Commander Gottos and reclaimed Castle Fynn.
Became allies with Ricard the dragoon.
Hatched the last wyvern egg.
The Dark Knight ascended the emperor's throne.
Past allies joined the rebel army.
Defeated the emperor and restored peace to the world.
Obtained Excalibur.
Learned all passwords.
Leveled up one weapon type to skill level 16.
Leveled up one magic spell to skill level 16.
Defeated 500 monsters.
Completed 10% of the Bestiary entries.
Completed 50% of the Bestiary entries.
Completed 100% of the Bestiary entries.
Obtained 10,000 gil.
Obtained 50,000 gil.
Obtained 100,000 gil.
Collected treasure from 10% of the chests.
Collected treasure from 50% of the chests.
Collected treasure from 100% of the chests.
Traveled to all the map locations.