there is a big ball with goo(?) on it, when i shoot it, it comes back. i cant get rid of it!!!! please help me!

I'm assuming it's the goo coming back you're talking about?
If it's the part I've just done, there should be a purple creature floating near you (look far left or far right) when you're sitting in the cannon - this is what's re-gooing the ball - shot it with the cannon to get rid of it. It re-spawns so keep an eye on it and just keep shooting it. Also, don't forget there's goo all over the ball so you need to shoot all the 'mirrors' around the ball to get the ball circulating so you can get rid of all the goo.