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.
Saw the Moolbeld Ruins.
Inflicted 1000 damage in a single hit.
Met Rhydderch.
Made one monster drop eight materials at once.
Fluxation occurred four times in a single battle.
Used combos in Momentum mode 30 times.
Ate Horse Oak Casserole.
Met Sayagi.
Defeated enemies with all Kill types.
Met Cornelius.
Saw the Apparth Ruins.
Fluxation occurred ten times in Command Spritnite.
Inflicted 500 damage in a single hit.
Met Hydor.
Used Momentum mode 100 times.
Met Akash.
Defeated ten enemies with Over Kill.
Used Momentum mode ten times in a single battle.
Met Freyja.
Made one monster drop four materials at once.
Defeated ten enemies with Exact Kill.
Met Raishin.
Met Hapsper.
Obtained all achievements.
Met Eutess.
Met the youth.
Saw the Laniff Ruins.
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 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.
Listened to all the youth's lines and defeated him.
Defeated the Ruler of Time.
Fluxation occurred ten times in Support Spritnite.
Fluxation occurred 200 times.
Added maximum Fluxes to a spritnite stone.
Used one megalixir.