To start the final boss battle, gather at least one ring. To fill your soul gauge, shake the remote up and down.
The boss will throw sparking balls at you. Fill your soul gauge at least halfway and then use time break to dodge this attack. The boss will come up to you and try to use a slash attack to the right, left, right, left.
Once you dodge that, he will throw more sparking balls. Use time break to dodge it and he will come up to you again. This time he will an X in the middle of the screen so move into a corner to dodge it.
After you dodge that, he will form this giant energy ball and hurl it at you. Make sure your soul gauge is full. When it is coming towards you, use speed break and shake the wii remote up and down. Sonic should kick the energy ball back at the boss.
Then charge your soul gauge enough to use speed break. Once you get to the lump on his body, shake the wii remote up and down again.
Once you push him back he will make a vortex on the left side, then the middle, and then the right side with his tail. So start in the middle and then move to the left after the left vortex moves past you. Do this whole process 3 times and you should beat the final boss.
Note: When you get farther into the battle, the sparking balls will be thrown faster and the boss will slash at you in different ways. Be ready for any attack and give yourself options.