Mad Skillz Achievement
Get all Skills.
First, make sure you get all red star rings in the required acts. This will unlock skills and automatically give them to you. Next, do all of the challenges. This will allow you to purchase more skills from the shop. Then complete the game with all seven Chaos Emeralds. All of the skills will now be purchaseable in the shop, but it will require a fair amount of rings, so make sure you save your money.