Here is the full list of all 16 Ys VI: The Ark of Napishtim achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Defeat Majuunun.
Obtain every other achievement.
Win a boss fight with 1 HP remaining.
Complete the game on Nightmare difficulty.
Acquire every Tabulas.
Win the duel with Geis.
Complete Boss Rush mode on any difficulty.
Complete the game in Catastrophe Mode.
Destroy the Ark’s Central Shaft.
Defeat Ernst.
Acquire all equipment.
Defeat Orjugan.
Defeat Ud-Meiyu.
Defeat Zonplas.
Win a boss fight without taking a single hit.
Defeat the Wandering Calamity.