A guide to unlock 符号42 Achievement in The Adventure of Magical Girl.
Defeating the Minotaur
The one and only thing you need to do to unlock the "符号 42" Achievement is to defeat the Minotaur. The only difficulty might be to find the cave where the Minotaur lives.
This cave entrance is North to some village (Mystery Village :? _ If someone can confirm).
I think it's Mystery Village but I'm not entirely sure, that's why I've taken some screenshots to give more specifics.
Exit that village to the West.
Follow the path, going North. You'll eventually end up in front of a cave entrance.
Enter that cave and search for the Minotaur. He's not hard to find, nor hard to beat (especially if you do this when your team is equiped and high level). Use magic :)