Dumbledore's first spell struck her right in the chest before she was ready. Dueling using cloaked magic was a step above where Hermione was comfortable. She had only just learned how to perform magic at all with the unfamiliar wand. How often did the wizard expect her to have to defend herself from attack? After a few more hits that hurt her pride more than her body, she discovered she was able to apply the principles he'd already taught her to the more active form of magic. Of course it made sense that she might have to duel an opponent. Spies were in almost constant danger.