Instructions
Kill ordinary Dementors using the Patronus Charm spell with Harry. Only Harry can perform this spell. Hold ̶0;Square̶1; on the Playstation 3 (or the ̶0;Attack̶1; button on other platforms) to bring up a target. Use the control stick to select the desired enemy. This automatically selects the correct spell to use on any enemy. Press ̶0;Attack̶1; again to use the spell. Keep your distance as much as possible when fighting Dementors. Alternatively, scroll to the correct spell using the ̶0;R2̶1; or ̶0;L2̶1; buttons on the Playstation 3.
Kill ordinary Dementors in this way during the Dark Tower level. In the boss fight at the end of the level, many ordinary Dementors will swarm around to make things more difficult. Kill them in this way, and use the Wingardium Leviosa spell on the barrels when you get the chance. A plunger will fall out of one of the two barrels on the level. The main Dementor will inhale in an attempt to pull your characters in toward him. The plunger will be pulled up instead and stick to his face.
Attack the Dementor with Patronus Charm when the plunger is stuck to his face. This makes him lose a heart. This is how you defeat him. To make it more difficult, the Dementor then freezes you and the barrels. Attack the statues near the barrels to melt the ice surrounding them.
Use Wingardium Leviosa to lift the barrels and get the plunger out again. When he goes to inhale again, the plunger sticks to his face. Attack him using Patronus Charm to remove another one of his hearts.
Repeat this process another time to finish the level. He will freeze the barrels again, so the statues should be attacked first. Then use Wingardium Leviosa and Patronus Charm to defeat the main Dementor.