You need to stand in one spot and then let him shoot his arms out and the attack the arms with the ice spell intil it freezes and then disappears. do that to the other arm and then when they are both gone you attack the head but only when it spots hermione (you need to use her for that part)and shoots out fire bits that is when you go up to it and use the ice spell on it.
I hope that i have helpped you in some way possible.