Q. What is Mastery?

A.

Mastery enhances your character in Attack, Defence, Skill, and Support.

It requires Mastery points to level up the mastery.

You can gain 1 Mastery points from every 3 level up,

and maximum Mastery points can be acquired is 166.

If you leveled up unwanted mastery, you can reset by using Diamond.

However, the reset applies on individual page, 

so if you reset Attack mastery page, Defence mastery will not be reset.