Azula would have probably had more warning about the approach of a furiously angry sixteen year old, but Penelo actually spotted her in the reflection of one of those large windows from around a corner.

And then there was a moment of inarticular mental keyboardmashing that could be summed up as 'She hurts all these people then has the nerve to come HERE?!?!?!'

So, a few moments later, she basically stepped around the corner, looking almost calm except for the fact that her hands are balled up into slightly shaking fists. And before the firelord can react, the normally calm Penelo has hauled off and punched her in the face. Surprisingly hard! Possibly hard enough to take her off her feet.