It wasn't the first time that Buffy had tried to punch him in the face but never before had she done so with such anger and intent. They had sparred many a time and sure she had gotten frustrated that she couldn't really hit him but never had she gotten this angry.
Lucius's reflex had kicked in before he had even realised what was going on but years of martial arts training and discipline had taught his body to react before his mind was fully aware of the threat. He managed to stop the punch from making contact with his face by stepping back on one foot and blocking with his forearm. The sheer force of the impact nearly broke his arm yet somehow survived it. He would bruise as a result, or at least he would if he allowed it and didn't just heal it. The pain was there of course but he paid it no attention, his body was used to this abuse and besides the pain would kick in his adrenaline.
"Then stay out of my business" Lucius told her coldly, centering his balance and moving into a defensive position automatically. Although he no longer had the qualms with offense as Daniel had, he was not foolish enough to make an attack until Buffy had worn herself out.