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