"No matter how subtle the wizard, a knife between the shoulder blades will cramp his style"

The message implies that also one of the most competent and also smart person, stood for by a wizard, can be hindered or beat by a basic and also unexpected attack, symbolized by a knife between the shoulder blades. In other words, despite exactly how smart or powerful one is, they are still vulnerable to physical damage as well as unforeseen events that can disrupt their plans and also capacities.