I don't understand Mastery

With the way the magic formula works, Magic Attack also adds a little bit to Mastery for every point. As you get more and more Magic attack, your mastery gets to be higher and higher causing a tighter damage range. This is why high-leveled Evan players that have 90% mastery have VERY tight damage ranges. The base mastery is 90%, and MATK only adds to that.

In a nutshell, for magicians:

INT:
Adds to MP regained per tick
Adds to MP per level
Adds a small, constant amount of damage per point
Base INT gives bonuses to Maple Hero
Includes all the benefits of MATK (because 1 INT also gives 1 MATK)

MATK:
Exponential for damage - each point in MATK is worth a little more in damage than the previous point
Adds a little bit to mastery per point

LUK plays no part in the magic damage formula.