Need help with Harry Potter and the Chamber of Secrets PS2 Game?

I'm stuck! I'm on the second day (Parsilmouth) and I'm in the defense against the dark arts class looking for a spell. I got passed the first part of the level with the spikey balls flying at me. Now I'm in the second part where you have to use another spell to cut down huge spikey balls into firey pits. So, I cut down the first spikey ball, but can't get the other one. I thought maybe it had something to do with the imps, so I managed to get it to a point where I had gotten rid of them all and then tried to cut down the ball. No luck. I've tried aiming and trying to cut it from different parts of the ramp. Nothing works and I'm stuck in that level. Anyone have any advice or secrets on how to cut down that second spikey ball? Please don't give me links to walkthroughs. I've looked through everyone and they all just say to cut down the balls.