"You want lookout duty, Malfoy, or am I watching our back?" It seems like Lucius' offense was better at range than hers; the best she can do before someone gets close is hurl a dart at them, but she has to have somewhere for them to go. ("Up" generally works; bless gravity.) But she also doesn't want to push the wizard wrong. He seems like he's full of cracks, and she has little interest in finding them all.