With early game equipment and skills the key is to be patient. Since troll caves have an overhanging entrance you can build a small pallisade in front and keep a fire underneath to give your character a resting bonus as you patiently go in and out of the cave to either hit it with range or melee attack (perhaps even with the sneak bonus) and get out before it can retaliate. As you progress, you will find troll cave entrances to be ideal for building temporary shelters as you journey through the Black Forest.