Hi maggsie,
Have you checked out any of the walkthroughs
Here. Hope it helps.
HERE'S a tip I read on gamefaqs.
Tip for beating bosses, see what spells they use on you and then use that spell on them ie. Voldemort uses impendimentia (sorry about spelling) so use that on him.
Cathy

EDIT....
Is this what you are refering too
BOSS FIGHT!!!
Giant
* * * * * * * * * *
Tap B twice to equip Impedimenta. Lock onto the Giant and then blast away with
it. The two knights in front of you will try to do battle with the giant. Spam
magical attacks at the giant as much as you can until Death Eaters arrive. Do
battle with the Death Eaters. They will stop appearing after the giant has
defeated both of the knights.
Backpeddal and spam magical attacks at the giant. Keep doing this until two
more knights intervene to help you. Keep repeating the process detailed in the
above paragraph until the giant's health is depleted completely and you're at
the end of the bridge. Then use Impedimenta to knock the giant off the bridge
and defeat him!
* * * * * * * * * *
BOSS FIGHT!!!
Giant
* * * * * * * * * *
All is safe...
OH WAIT NO IT'S NOT!
* * * * * * * * * *
BOSS FIGHT!!!
Giant
* * * * * * * * * *
Snipe the giant from at the end of the bridge. Kill the Death Eaters as they
come. Use the shield as much as possible. Don't bother shooting the boulders
out of the sky that the giant throws. Just dodge them. Then return to attacking
the giant with a variety of spells from a distance. Keep doing this, defeating
the Death Eaters, and spamming magical spell attacks at the giant.
The giant will eventually get close enough to do battle with a pair of knights.
The giant will fight with the knights, and that's your cue to drain the
remainder of his health bar. When it's at zero, tap B twice and then fire the
Impedimenta spell at it to defeat it.
* * * * * * * * * *
BOSS FIGHT!!!
Giant
* * * * * * * * * *
The Death Eaters will then charge from the bridge. Defeat it from your current
location with sniping spells and Confringo. When enough of them have been
defeated, a QTE will trigger. Press Y when prompted. Then tap on LB repeatedly
for the next step. Finally, press Y one more time to send the giant falling
back down into the water below.