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