Here is the full list of all 22 The Legion of Maelstrom achievements.
It begins...
Completed battle training.
Defeated "Immortal-One."
Repair the bridge.
Unlocked class-change.
Allowed access to the ship.
Link Soldier's ability to all classes.
Link Priest's ability to all classes.
Link Ninja's ability to all classes.
Link Wizard's ability to all classes.
Upgrade any class armor.
Upgrade potion effects.
Got a [Rare] or [Epic Rare] Gachapon prize.
Unlocked the Ocean seal.
Unlocked the Graveyard seal.
Unlocked the Forest seal.
Unlocked the Desert seal.
Deafeated Maelstrom as the Soldier.
Defeated Maelstrom as the Priest.
Defeated Maelstrom as the Ninja.
Defeated Maelstrom as the Wizard.
Defeated Maelstrom as the Citizen.