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