When you beat the game you get a Harry Potter card. You can use the card to give someone all their spells. The cheat is you can use the card unlimited times.

Before having Ron use Spongify on the sponge/spring, have one of the characters perform Flipendo on the right side of the really big rock. The rock will then move into the hole you see on its right. When the characters spring up and over where the fourth book was, just go up to the fifth one.