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