Grace wasn't sure exactly what sort of thought bloomed in her mind as she threw the punch - maybe she was thinking about just punching him, or maybe she went as far as to think about throwing him against the window behind him, but whatever the case, she ended up doing way more than that.
The moment her hand moved forward, Rune moved at the same time, seeking to dodge the punch just like he had been doing all this this time. However, it wasn't her fist that he needed to defend against, but the surge of force that took him by complete surpise and threw him against the glass window.
The force was so strong that it not only threw him against the glass, but broke through that very glass as well, taking him down with it.
Everything happened so quickly that Grace barely got any time to register it. One moment she was throwing the punch, and the very next moment, she caught a glimpse of Rune falling down the building through the now broken floor-to-ceiling window.
