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