Lucius Malfoy stood frozen, staring at the elf. Then he lunged at Harry.
"You've lost me my servant, boy!"
But Dobby shhouted, "You shall not harm Harry Potter!"
There was a loud bang, and mr. Malfoy was thrown backwards. He crashed down the stairs, three at a time, landing in a crumpled heap on the landing before. He got up, his face livid and raised his wand, but Dobby raised a long threatening finger.
"You shall go now," he said fiercely, pointing down at Mr. Malfoy. "You shall not touch Harry Potter. You shall go now."
Lucius Malfoy had no choice. With a last, incensed stare at the pair of them, he swung his cloak around him and hurried out of sight.
Anyway, magic seems to come to house-elfs more easily. They might work in their advantage in a fight against a wizard.