I understand about ethical hunting, but defending yourself has nothing to do with that. Shooting something several times is fine for self defense, not so much hunting.
My biggest questions is of course penetration through dense muscle and bone or penetrating the skull of something with its head down coming straight at you. A nice shot through the lungs or both shoulders is not really the question. That is a hunting shot not a shot you get from a charge.
Lots of large bears have been killed with slugs. And please note I said slugs not slug. But while slugs do a lot of damage I would be surprised if they get much penetration as they flatten out. So, yeah, cape buffalo, rhino and elephant I figured from the start was out of the question. But I also figured if you can kill a bear with a slug it would kill a lion. They are quite a bit larger and generally pretty well armored with fat and thick skin. But like I said, all conjecture.
Its all well and good to talk about a double rifle chambered in .416 Rigby, but who actually owns one and its a bit expensive to get one just for a trip to the woods. But just about everyone owns a shotgun.