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 unlocked all achievements.
You learned all of Gau's "Rage" commands.
You defeated Omega Weapon.
You completed the Soul Shrine.
You collected all the Magicite.
You completed the Dragons' Den.
You had all of Shadow's dreams.
You finished the game.
You learned all of Strago's "Lore" commands.
You discovered all of the hidden "Blitz" commands.
You found and befriended Locke.
You brought all the characters together.
You learned all of Cyan's "Bushido" commands.
You accumulated over 1 million gil.
You found and befriended Relm.
You racked up 10 wins at the Coliseum.
You found and befriended Umaro.
You found and befriended Gogo.
You learned all of Sabin's "Blitz" commands.
You got all of Edgar's Tools.
You found and befriended Mog.
You defeated Deathgaze.
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 matched all the symbols on Setzer's "Slots."
You discovered the truth about Terra and her hidden power.
Thank you for purchasing the game!