Any Hero has maximized one modification of one aspect.
Firstly, you must have the right skills to obtain at least three points to fully mod one skill. By putting enough points into a certain class' skill "combat mastery" you will obtain more and more modification points used to modify the skills of that "combat art tree". Each class starts off with one skill already on the specific "combat art tree". Once that "tree" has been leveled enough in the skill screen (you level the skill by gaining levels), you can then spend modification points on say, the skill that came with your character. To do this, open up the "combat arts" menu, and press "modify skill". You will see 6 dots. 3 Can be chosen to modify the skill. Once the third is selected, the achievement will pop.