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