Here is the full list of all 12 Little Fire Girl Fights Final Boss / 小火女掉站终极Boss! achievements.
Reached the final phase of Void Demon
Defeat both Bosses!
Defeat Void Demon with only blasts and meteors.
Defeat Void Demon with full health remaining
Defeat Ice Priestess with full health remaining
Defeat Ice Priestess without any casting any AOE spells
Defeat Void Demon in under 5 minutes
Defeat Ice Priestess in under 5 minutes
Clear 10 waves of the Challenge
Casted every basic spell combination at least once
Defeat the Void Demon
Defeat the Ice Priestess