My money is on the brown bear, one swat of those paws and anything is in some deeeeeeep trouble.
No doubt, a brown bear wins that battle. Also, I really don't see why a 45/70 or 450 Marlin wouldn't be great medicine for big cats. Lions are relativley light skinned animals compared to bears. I would think a great big 45 bullet would slice through a lion or leopard like a hot knife in butter.
I know that lions and leopards are very quick when they attack, but I would think a lever would have an advantage over a bolt when it comes to speed of operation getting additional shots off.
American Mountain lions are killed with 30/30's and pistols. They are not that tough when hit. Why couldn't a big bore levergun work on their larger relatives?
I do understand the preference for "super powerful" big bore bolts when it comes to the other dangerous African game (as they are thick skinned and muscled). I would agree that a 45/70 or 450 would be marginal.
I just don't understand why a big bore levergun wouldn't be a good choice for Lions & Leopards. I understand that most African hunters are partial to bolt actions, but I think the dismisal of these leverguns makes no sense - its irrational.