Regarding bear attacks, the saying goes 'If it's white, good night', but what would be a reasonable thing to do in case of a polar bear attack?
The thing that marks polar bear attacks apart is that it is never about territory, they're not very territorial. Although they are risk averse, they don't truly fear humans - they are a true apex predator.
Therefore, polar bears attacks are almost always about lunch. And as polar bears sometimes need to go days without food, they like to stock up when the opportunity arises. This is bad news for you.
Fortunately attacks are rare, happening only 1–3 times per year. But they do happen. As with every bear encounter, the one you prevent is the one you are most likely to survive.
You NEVER go into polar bear country without precautions. A firearm is good at long distance but bear spray is the best bet for close range. The bear can close distance quickly. A third option is to bring a flare, although flares are short term deterrents.
Otherwise you bring a well working snow machine or other means of escape. As a last ditch effort, you bring somebody who runs slower than you, and whom you don't really like anyway.
As with any large beast, if it's just mano a carnivore then go for the nose and eyes. They're on the head, positioned 3 feet above you.
The following links go into more depth about attacks.