Trolls are actually very easy to parry with a shield (banded or bronze or better). A trolls swing is broadcast because it is so slow, make sure not to raise the shield to early or it will just block, raise it as the trolls broadcasts it strike (begins its swing), and you will stunlock it every time. While stunlocked one can deliver at least 2 blows, or one very damaging secondary attack. I guarantee you, you will be surprised at how easy it is to parry and stunlock a troll. I have not died to a troll or multiple trolls since my young nephew showed me how to parry their attacks with shield. As a previous post said, it is a good idea to crouch when entering and sneak up behind the troll for your first blow to do more damage.