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