Here is the full list of all 50 I am Setsuna achievements. It takes around 30-35 hours to unlock all of the achievements on Windows.
Obtained all achievements.
Listened to all the youth's lines and defeated him.
Met the youth.
Defeated the Dark Samsara's final form.
Defeated the Ruler of Time.
Defeated all the Spritnite-Eaten Monsters.
Fluxation occurred 200 times.
Fluxation occurred ten times in Support Spritnite.
Obtained the Dragnarok spritnite.
Obtained the Doom spritnite.
Obtained the Rebellion spritnite.
Obtained the Eclipse spritnite.
Obtained the Gearadia spritnite.
Obtained the Genesis spritnite.
Defeated all members of the Pengy species.
Sailed on the exploratory ship.
Obtained the Holy Light spritnite.
Met Eutess.
Inflicted 1000 damage in a single hit.
Defeated all members of the Hoot-Hoot species.
Used one megalixir.
Saw ???.
Ate Nive Vegetable Stew.
Ate Starberry Cake.
Saw the Laniff Ruins.
Saw the Moolbeld Ruins.
Added maximum Fluxes to a spritnite stone.
Met Rhydderch.
Ate Gatherington Fries.
Ate Gaia's Blessings Stew.
Fluxation occurred four times in a single battle.
Inflicted 500 damage in a single hit.
Met Sayagi.
Ate Horse Oak Casserole.
Met Cornelius.
Saw the Apparth Ruins.
Met Hydor.
Met Akash.
Defeated enemies with all Kill types.
Made one monster drop eight materials at once.
Fluxation occurred ten times in Command Spritnite.
Used combos in Momentum mode 30 times.
Used Momentum mode 100 times.
Defeated ten enemies with Over Kill.
Met Freyja.
Used Momentum mode ten times in a single battle.
Made one monster drop four materials at once.
Defeated ten enemies with Exact Kill.
Met Raishin.
Met Hapsper.