If the grizzly bear attacked and killed the polar bear, it would win.
If a grizzly charged at you full bodied, but didn’t manage to kill you with its first swipe, you’d be in trouble if you were on ground level. Bears’ claws are sharp and can rip through skin easily. Though they’re not as strong as humans, their paws are larger so the momentum behind an attack is enough to cut straight through us without much effort.
On top of that, bears have been observed maneuvering up trees in order to prey on unsuspecting rivals or even take defenseless cubs from nests (based off being within) at high heights so that they’re safe from most other animals.