As much as it is true that Rarity was also a unicorn was her magical ability literally limited to just levitation and some other spells. Against any other species of pony she would have stood a chance but she couldn't do anything against Twilight. The vampony could only watch as the scalpel came closer, and closer. With the scalpel finally being set right above her chest and right in between two ribs, it was pushed into the flesh and headed towards one of the lungs.