If possible, you want to also start fights with trolls by shooting them when they're unaware. You can get a big damage increase on them, and start the fight with a big chunk o health gone already.

I then finished them off by kiting them around the big trees in the black forest, and shooting them in the face with a bow. If the bow is strong enough, it staggers and stuns them for a second, allowing you room to break contact and get farther away. Then you just keep shooting them in the face.

I'm pretty sure from seeing this in another thread, but shooting them in the head doesn't give any damage bonus, but it did seem to stagger them more often.