Here is the full list of all 23 Maiden and Spell achievements.
Clear a True Magnus
Defeat the Heroes United.
Defeat the Collector of Stars.
Complete the Sun Priestess's Adventure
Complete the Silent Redhood's Adventure.
Defeat Lightning Magnus: Phantom Voltage in under 21 seconds.
Complete the Royal Arcanist's Adventure.
Complete the Hero of Frost's Adventure.
Complete the Hero of Frost's Adventure on Very Hard.
Complete the Royal Arcanist's Adventure on Very Hard.
Complete the Silent Redhood's Adventure on Very Hard.
Complete the Sun Priestess's Adventure on Very Hard.
Defeat the Collector of Stars on Very Hard.
Defeat the Heroes United on Very Hard.
Clear all True Magnuses.
Perfect all True Magnuses.
Survive Fire Magnus: Dragon's Eye for 60 seconds on Very Hard.
Defeat Life Magnus: Anima in under 17 seconds.
Defeat Steel Magnus: Epic Steelfall on Very Hard without taking damage.
Get hit with all 6 status effects at once during Dark Magnus: Metronome.
Survive Water Magnus: A Bigger Fish for 80 seconds on Very Hard.
Defeat Light Magnus: Lay On Hands on Very Hard without taking damage.
Follow instructions as the Silent Redhood.