Here's a link to a guide I made that details the location of everything needed for this achievement.
http://www.gamefaqs.com/xbox360/631934-lego-harry-potter-yea...In order to obtain 100%, you need to do the following:
Obtain all 200 Gold Bricks
- 24 for completing all story levels
- 24 for getting True Wizard in all story levels
- 24 for finding all 4 Hogwarts crest pieces in every level
- 14 for completing all the lessons during the story
- 60 for finding all 60 Students In Peril
- 16 can be purchased from Borgin and Bukes
- 1 for completing the bonus level in Borgin and Bukes
- 37 can be found in the Hogwarts/Diagon Alley/Wilderness areas
Collect and buy all 20 Red Bricks
Collect and buy all 200 characters
- 72 in story levels (3 per level)
- 128 in the Hogwarts/Diagon Alley/Wilderness areas
Edit: If you end up with a mysterious missing gold brick, go here
http://support.wbgames.com/link/portal/24022/24028/Article/2... and use the codes and it should pop your related achievements. Thanks Triblade for the info.