When the first set of fire comes down, turn into your Morph Ball and go all the way over to the fire ball on the right. Wait until it starts to go to the middle. Then, keep shooting missiles (when the spider is charging up) until it shoots the three fire balls. You will get hurt, but at least you will hit him. You have to keep going next to the fire balls until he loses his legs. Once this happens, you can go to the right of the stage and shoot missiles up at him. When he turns red, go to the left of the stage and shoot up.