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