@Malos it's actually quite easy to use: there is a magnet behind every wall, so by turning them on and off, you can control the direction of the magnetic field, making the knifes fly in any direction you want.
As explained in a previous chapter, skills are granted to those that first complete a dungeon and that "cursed skill" is a skill granted only by 1 particular dungeon so the effect cannot be reproduced, unless multiple dungeons can grant similar skills
@Aretheus the girl didn't find a way to accelerate a person growth. The landlady's curse was slowing her growth so the girl used her power to reverse the effect. She can't do the same thing to anybody else.
About our MC, yes, his ability is broken but he had to practice a lot to get it.