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