Here is the full list of all 44 Valfaris: Mecha Therion achievements.
Defeat the Jelly Queen.
Defeat Anvin.
Defeat Kykris.
Defeat Tycrid.
Defeat Carnack.
Defeat Vykham.
Defeat Gorgoroth.
Defeat Herrick without letting Xyla take any damage.
Defeat Hern without letting Xyla take a hit. Demigod (Normal) mode or higher required.
Defeat Garrex.
Defeat Valkyrie and Kalmah.
Defeat Zharn.
Defeat the Heart of Jelly.
Defeat Vroll.
Find your first weapon.
Find Draconis.
Find Eris Ice Axe.
Forge the Soul Eater.
Find every weapon.
Find your first module.
Find every module.
Upgrade a weapon for the first time.
Fully upgrade a Destroyer weapon.
Fully upgrade a Melee weapon.
Fully upgrade an Auxillary weapon.
Find your first Blood of Valfaris.
Find 10 Blood of Valfaris.
Defeat a Champion.
Defeat 20 Champions.
Die for the first time.
Meet Xyla.
Get 666 kills.
Destroy every Cerberus Wheel in the Ruins of Anvin.
Destroy all sections of a Fenrir Class Destroyer.
Finish a stage without taking any damage. Demigod (Normal) mode or higher required.
Destroy 25 Fuel Canisters in a single playthrough.
Kill all of the Spy Fleas in a single playthrough.
Discover a secret area.
Discover every secret area.
Finish the game in under 2 hours. Demigod (Normal) mode or higher required.
Finish the game with 10 or fewer deaths. Demigod (Normal) mode or higher required.
Finish New Game +.
Defeat Vroll on Great Horned One (Hard) mode.
Defeat Vroll on True Metal mode.