Charlie owled back to Harry that few things other than a Patronus could drive back a Dementor, but if he had the help of at least one of the twins he could probably devise something.

There must be something, because in HBP Harry fails a DADA essay because "he had disagreed with Snape on the best way to tackle dementors," but I can't for the life of me figure out what it is. If it was easier, Lupin would have told Harry about it rather than the Patronus, right? Or in addition. Or . . . something.