Here is the full list of all 28 Final Fantasy VI (Old ver.) achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
You collected all the Magicite.
You completed the Dragons' Den.
You learned all of Strago's "Lore" commands.
You finished the game.
You matched all the symbols on Setzer's "Slots."
You discovered all of the hidden "Blitz" commands.
You got all of Edgar's Tools.
You had all of Shadow's dreams.
You accumulated over 1 million gil.
You brought all the characters together.
You found and befriended Locke.
You found and befriended Gogo.
You defeated Deathgaze.
You learned all of Sabin's "Blitz" commands.
You found and befriended Umaro.
You found and befriended Mog.
You learned all of Cyan's "Bushido" commands.
You found and befriended Relm.
You racked up 10 wins at the Coliseum.
You opened more than 80% of all treasure chests.
You witnessed the World of Ruin event.
You continued to defeat Ultros to the bitter end.
You discovered the truth about Terra and her hidden power.
Thank you for purchasing the game!
You unlocked all achievements.
You learned all of Gau's "Rage" commands.
You completed the Soul Shrine.
You defeated Omega Weapon.