I have killed trolls in their caves with a basic finewood bow and lots of fire arrows.

There is not a lot of room for ranged attacks inside a troll cave. Neither is there a lot of room for the dodging required for a melee assault if one does not have the food, block skill and gear required to be able to block a troll's attack.

The first time I enter a troll cave, the troll is usually at the very back. One has to sneak about halfway in to catch sight of it. Two perhaps three bow shots, then run back to the entrance. If one is quick, one can turn and get another bow shot at the troll bearing down.

Then exit. Wait 20 or so real time seconds. Reenter the cave. Most of the time, the troll will be on his way back to the far end of the cave. Two quick shots and run. Even if the troll has not attempted to go deeper into the cave, one can usually get off one quick shot and run.

Yes, it is tedious and it does take a lot of game time. I don't do it unless I really, really need troll hide and cannot find a troll roaming in the Black Forest. Much less tedious to get the food, gear and sharpen the block and spear skills before going after trolls in their caves.

Why spear? Attack is faster than other melee weapons and trolls are weak to piercing.