you have to cast flipendo on him untill i falls in to the big HOLE!! to the left!

Hi i hope that this will help you. All you have to do is hit the spongify spot on the ground in the center of the room. Then quickly run up to the spot and stand next to it....but don't walk into it. Make sure before you do this that the troll is running towards the spongify spot. Then the troll will try to hit you but luckily you are standing in the right spot and the troll will hit the spogify area instead.If you are standing in the right spot then the troll will spin his club in the air before hitting the spongify area...this will give you time to run away from his club. His club will come back and hit him on the head when he hits the spongify spot. You will have time to run from him after this. It will take five times to kill him......GOOD LUCK!!!

This is actually very easy. When you are in the room with the troll, there are lots of small holes on the ground. Harry will fall in these and die, but the troll will walk right over them. However, if you go over far enough to the left (while avoiding the troll and his club) there is a very large hole. The troll will fall down this hole. What you have to do is hit him with Flipendo when he is raising his arms and roaring, and he will slide backwards. If you are lined up and aim properly, he will slide back into the hole.