Only encounterd one, and that was up the mountain... (wont spoil, but pretty early in the game) just had to take a slightly different route to avoid him. Regular trolls are also nasty bastards, they can hit quite hard...
Snow bears are quite tough, but maybe they aren't and just have a huge HP bar. The bear did take down my companion though and I could only kill it with an insanely long ranged battle (got the bear behind a rock, and just arrowed it).