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