You need to have all gold bricks, all characters and all red bricks unlocked. You also need to finish the bonus level in the shop at the end of Knockturn Alley. If you are stuck at 99.9%, just go into the red brick shop and check if you actually bought all the bricks (just unlocking them is enough for the red brick trophy, but will not get you the %). Same goes for the characters - check if you actually bought them at the shop.