In Diagon Alley, buy 30 pairs of boots. Then when crossing the moat to the castle, pause and unpause the game, and you'll be able to press the button to shoot owls at the bugs flying over the water.

Beat the game and save, you'll be able to start a new game with the same experience, cards and sickles you collected. This makes it alot easier to beat the game again and get to MAX level.

Collect all 15 Folio Magi cards and Ron will give you the final one which will unlock a secret ending on completion of the game.