How do I hurt Voldemort at the end of Year 5?

I've reached the part of "A Veiled Threat" where Dumbledore joins the fight. I've managed to knock a couple of Voldy's hearts off — I shot him, then destroyed his fire dragon — but now he's standing on the fountain where I can't shoot him or jump up to him.

I've got a large ball of water in the fountain which I can move a little by shooting, but it resets frequently and I can't get it very far before Voldy kills me. Trying to jump on it just pushes me back. How do I continue the battle?

This video's probably the most helpful, skip to about 10:45 mark to see the part you're talking about:

It appears you can lift the ball of water and throw it at Voldemort, which will cause him to change tactics.