Remember when Sam killed a white walker with dragonglass. He kinda blindly stabbed, grazed the walker, and then the walker shattered into a jillion icy bits. The same thing happened when Jon killed one with Longclaw's Valyrian steel blade.

So it doesn't appear that the location of the blow, the power of the strike, or the severity of the injury means a damn thing. All you gotta do is score a hit, and some magical reaction makes the bad guys explode.

Jon's got a mountain of dragonglass. How does ice-dragon stand a chance? One volley of arrows and Viseyron-z becomes nothing more than a rain of ice cubes, no?