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