When doing the CPU battle, you can die and restart as many times as you want. You only have to beat them all without exiting the game, if you do it will restart you back at the first CPU opponent. I set my attack mode to H. Although a few times I did beat them using the one that is all rainbow bubbles. Once you beat an opponent and it has the red slash through it you'll move on to the next one. Keep going until all 8 are defeated and the cheevo will pop. My total clear time was 22 minutes and change. My actual play time was a little over an hour because I kept dying on level 4.