We use level 3 holsters where I work, but we were taught to carry a knife clipped to your left pocket and if someone is grabbing at your gun, you grab their wrist with your right hand, and start thrusting the knife into their face. It sounds really brutal but it's supposed to be a really good way to make someone immediately pull their hands up to their face. Obviously at that point, you quickly assess whether to shoot them or not and go from there. Obviously wouldn't work on the ground, but would have worked in the case of this story.
"We have repeatedly held that one who is unlawfully assaulted and put in apparent danger of his life or of great bodily harm need not attempt to escape but may repel force with force..." PEOPLE v. BUSH
414 Ill. 441 (1953)
Supreme Court of Illinois.