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