I've completed the game so listen up!
At the end of the game you'll have to fight some Knights the light silver ones they
are easy to kill just use your knockback jinx.
However the darker ones are slightly harder when they swing their axes, about a 1/2 of a second after they'll leave thier bodies open to your jinx (fully charged.)
Voldemort will run away drink the Wiggenweld potion that is left and then follow him.
All you have to do is hit the pillars so they fall on him, thencharge your jinx, fire it into the mirror and when he is stood level with it fire again.
The rest is easy press X repeatedly to fight for life and square to hurt him when it says...