Just how hard is it to kill a Troll?

alright here are my ideas...

Mountain (cave) trolls as far as i know don't turn to stone, only the forest trolls do
(but i'm not sure on that one)

And am i the only one who thought the trolls in the movies were too small?
maybe i just have a big imagination.

Also i got the impression that forest trolls were as wide around the waiste as tree trunks

But then again, i have only read the hobbit, LOTR and half of sil