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