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