Complete the last level of Year 3 "The Dark Tower". The final fight with the Dementors can be difficult, however the strategy is to empty the 2 barrels, then wait for the Dementor to use levitate against you. Once the items on the ground strike him he will become vulnerable to your Patronus charm. When he begins freezing the barrels strike the two statues near them to melt them. Three hits will kill him.