Perhaps they are alike to Dragons, not hard to kill if you know how. Dragons were Week in the under belly, Smoug and Glouring's deaths are good examples of how one could do so. Until I realised Frodo stabbed the foot of the troll I often assumed it was so.
But then you must realise that there are many types of Trolls, mountain, forest and Cave trolls. So perhaps some are easier to kill then others. The trolls that the Dwarves and Gandalf dared not fight were mountain Trolls, SO perhaps they were stronger and harder to kill. Plus I don’t think Gandalf could have taken on three Trolls at once on his own, even with his power and the ring he bore.
But then again, this is an area in which many are confused by what happens in those a cursed films, As in the book Frodo simple stabs at the feet and the troll retreats.