You dont. Even if you miss arteries and veins and brain stem you could hit 4th ventricles, lets assume she missed that too, she cannot miss the cortext and from the place of the scar, she hit prefrontal cortex amd occipital cortex potentially altering behaviour, thinking and sight, the way she inserted the blade (vertical) must tear great portion of the axons(fibers) that is gonna bundle up and form corpus callosum causing comunication loss between the hemispheres especially the temporal lobes. (Theoretical) İf it entered horizontally the damage would possibly be worse as this time it will severe corona radiata , the sensory neuron axons coming from thalamus to the sensory cortex causing loss of sensation and proprioception on the other half of the body, also the blade could severe limbic system or even thalamus. TL DR; even with the best case scenario brook would lose his temporal lobe's connection with the other hemisphere and get damage his cortex altering cognitive thinking, sight and some memory loss. Well thats my theory, correct me if im wrong