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.
Fluxation occurred 200 times.
Defeated the Ruler of Time.
Saw the Laniff Ruins.
Sailed on the exploratory ship.
Defeated all the Spritnite-Eaten Monsters.
Saw ???.
Saw the Moolbeld Ruins.
Fluxation occurred ten times in Support Spritnite.
Inflicted 1000 damage in a single hit.
Ate Nive Vegetable Stew.
Ate Starberry Cake.
Defeated all members of the Hoot-Hoot species.
Obtained the Dragnarok spritnite.
Obtained the Eclipse spritnite.
Obtained the Doom spritnite.
Obtained the Rebellion spritnite.
Obtained the Gearadia spritnite.
Added maximum Fluxes to a spritnite stone.
Defeated all members of the Pengy species.
Fluxation occurred four times in a single battle.
Obtained the Holy Light spritnite.
Obtained the Genesis spritnite.
Met Eutess.
Used one megalixir.
Ate Gatherington Fries.
Ate Horse Oak Casserole.
Ate Gaia's Blessings Stew.
Met Rhydderch.
Used combos in Momentum mode 30 times.
Fluxation occurred ten times in Command Spritnite.
Met Sayagi.
Made one monster drop eight materials at once.
Met Cornelius.
Saw the Apparth Ruins.
Inflicted 500 damage in a single hit.
Defeated enemies with all Kill types.
Met Hydor.
Used Momentum mode 100 times.
Met Akash.
Defeated ten enemies with Over Kill.
Used Momentum mode ten times in a single battle.
Met Freyja.
Defeated ten enemies with Exact Kill.
Made one monster drop four materials at once.
Met Raishin.
Met Hapsper.