Professor Longbottom: Let us get a few things perfectly clear, ladies and gentlemen! Standing up to your opponent and shouting a challenge sounds very good in stories! Engaging in a duel, wand to wand and face to face, makes for excellent photographs! But you will not be fighting in stories or photographs, ladies and gentlemen! You will be fighting in a war, against wizards who do not care that you are young and who do not care about fair play! They care only that you are fighting back, and they will hurt you and kill you if they get the chance! Therefore, your primary job is to not give them that chance!
She follows this up with three very simple rules of battle:
1. Keep your friends alive.
2. Get your objective.
3. Don't die.
Hermione and Draco manage to steal Hufflepuff's Cup from the Lestrange Gringotts vault without getting caught.