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