Defeating Arog the Giant Spider:
In the Dark Forest, circle around him and cast Rictosempra.  He will not shoot any spider 
webs at you. 
Submitted by unknown

Neville
When you have to rescue Neville from behind the tapestry,  go out throught the fat lady's 
picture and stay at the wall.There is a bookshelf to the left with a crawl through at the 
bottom. This takes you to a staircase leading to the fourth floor. Then you don't have to 
try and get past Percy. Then, on your way to the library, there is prefects. Cast a spell 
away from  you and the door to the library  and that gives you time to run to the library 
door.  In  the library,  run straight  across the room  and get the book. Then go out the 
library and do the same thing with the prefects. Then go to the entrace hall. Stay close 
to the wall.At this bit it helps if you have stink pellets.Cast a spell away from you and 
run to the door. If any of the prefects come near you, throw a stink pellet at them. 
If anyone knows how to get past the prefects in the grounds and to the greenhouse, let me 
know! 
Submitted by stephanie.grant at ntlworld.com