Wow! Great pick! I was kicking around the idea of a "Harry Potter characters in the Buffy universe" fic idea, but I couldn’t figure out which adult to use as Giles - now you’ve solved it for me! :)
You can’t see it because they’re on "our side" of the picture, but Harry and Co. are squareing off against Lucius as Ethen Rayn, Narsissa as Darla, Voldimort as the Master, and Belatrix as Drusilla (or possibily as Evil!Faith) with the death eaters as misc. demony minions.
Not currently on scene are Remus as Oz (who, because it’s the full moon, has locked himself in the library book cage), and Sirius as Angle with Kingsly as Gun who are tearing down the highway to the rescue, leaving Cho as Cordelia suffering from a killer headach back at the Hyperion in the care of Luna as Fred.