Here is the full list of all 33 Ys: The Oath in Felghana achievements.
Defeated "The Deadliest Cuteness."
Beat the game on Nightmare.
Beat the game on Inferno.
Escorted Elena to safety on every difficulty level.
Escorted Christof to safety on every difficulty level.
Defeated the final boss without taking any damage.
Achieved max level.
Beat the game in under 5 hours.
Conquered Boss Rush on Normal.
Beat the game on Very Easy.
Beat the game on Easy.
Beat the game on Normal.
Beat the game on Hard.
Maxed out all equipment.
Acquired every jewel.
Encountered every enemy in the game, excluding bosses.
"Killed" 50 immortal beasts without using the Silver Chimes.
Killed 500 Selnades across all playthroughs.
Spent over 10 hours in a single playthrough.
Spoke with every character in the game at least once.
Sold Randolph's ring to Cynthia.
Saved all of Hugo's pikkards.
"Measured" Elena.
Completed Anya's treasure-hunt game.
Defeated 5 or more enemies (except Mozgouz) at the same time.
Killed 2,000 Mozgouz across all playthroughs.
Died over 10 times on a single boss.
Enabled "No-Fall" mode.
Began the game on Very Easy.
Conquered Boss Rush on Hard.
Conquered Boss Rush on Nightmare.
Bested the master of the mountains in one-on-one combat.
Completed all other achievements.