A polar bear would destroy the shit out of anything short of an Elephant or Rhino, or possibly a hippo/salt water crocodile.
they are the biggest land predators, their jaws are so big they can literally hold a basketball in their mouth (which would be instantly popped by 3 inch teeth), they are really fast, really aggressive, they are enormously strong, and they have giant sharp ass claws. Also, their naturally long fur and thick skin and fat layer would also provide naturally tough armor.