Aule's Dwarves, before they were endowed with 'souls' of their own, were like brain-controlled robots. Think that it was said that whenever Aule would look away from them, they would stop and await new commands.
Surely Melkor would need to think elsewhere at times during a battle, which would leave his robots vulnerable.
That said, what about the whisperings of Saruman? Again, I don't have the quotes, but wasn't it said that, even when far away, some still heard the whisperings of Saruman in their ears, compelling them to do his will?
Turn that up a notch with Melkor, and you could have zombies.
