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.
Obtained the Dragnarok spritnite.
Obtained the Eclipse spritnite.
Sailed on the exploratory ship.
Obtained the Genesis spritnite.
Saw ???.
Defeated all members of the Hoot-Hoot species.
Defeated all members of the Pengy species.
Ate Nive Vegetable Stew.
Ate Starberry Cake.
Saw the Moolbeld Ruins.
Ate Gatherington Fries.
Saw the Laniff Ruins.
Obtained the Doom spritnite.
Obtained the Gearadia spritnite.
Obtained the Rebellion spritnite.
Fluxation occurred ten times in Support Spritnite.
Obtained the Holy Light spritnite.
Met Eutess.
Used one megalixir.
Met Rhydderch.
Added maximum Fluxes to a spritnite stone.
Ate Gaia's Blessings Stew.
Used combos in Momentum mode 30 times.
Met Sayagi.
Fluxation occurred four times in a single battle.
Fluxation occurred ten times in Command Spritnite.
Met Cornelius.
Ate Horse Oak Casserole.
Used Momentum mode 100 times.
Saw the Apparth Ruins.
Inflicted 1000 damage in a single hit.
Inflicted 500 damage in a single hit.
Met Hydor.
Made one monster drop eight materials at once.
Met Akash.
Defeated ten enemies with Over Kill.
Made one monster drop four materials at once.
Defeated enemies with all Kill types.
Used Momentum mode ten times in a single battle.
Met Freyja.
Defeated ten enemies with Exact Kill.
Met Raishin.
Met Hapsper.