Expecto patronum
When you get to the very last bit of the game a whole load of dementors will go after sirius black they wont be looking at you.
You have to power up expecto patronum shoot it at the dementors before sirius blacks health drops, then you have completed the game.
PS: the level is very hard.
More tricks, tips and cheats for this game are right here - Harry Potter and the Prisoner of Azkaban cheats
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