You upgrade your skills along the Road to Rule. You can open the chests as you progress or in the last area, just open the last level 5 chests.
If you miss any before the last chest, it makes the level 5 chests the price of all chests for that skill you did not open.
So, open the 3 level 5 skill chests at the end of the Road to Rule and you will get he achievement.