While duelling other wizards, or the gargoyle in the expelliarmus challenge, when you're sending the expelliarmus ball back, use Diffindo(cutting spell) on the foe. While he's jumping up and down saying "Ow! Ow!", then he won't be able to block the expelliarmus ball, and will lose a point. Do this a few times(you'll get the hang of it), and you should win.