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