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