To defeat a Roto-Disc you will need to be wearing a Tanooki Suit. After you have got a Tanooki Suit, find a Roto-Disc and make it so that it is underneath you. Right before it hits you quickly turn into the statue by holding the Down and B button. Once it hits from underneath you as a staue, the Roto-Disc will be defeated. Incase you don't no what a Roto-Disc is, it's the diso balllike enemy that rotates around a metal ball.