There is an easy way to kill trolls in the beginning. Cut a bunch of trees so that the ground is littered with logs and then just lure the troll over. Trolls can´t step over logs so if you retreat behind the logs he can´t reach you while you can shoot arrows at him. Deal with the greydwarfes beforehand so they don´t annoy you.

Oh almost forget another good advice: Don´t run in the blackforest. The sound of your footsteps draws in all the mobs from far away. You don´t even need to sneak. Just normal jog and you will notice a huge difference in the amount of enemies that notice you.

As a general rule keep distance to trolls in the beginning, they hear very well. Greydwarfes usually notice you first. Lead them away from a troll and kill them first, then deal with the troll by luring him around thick clusters of trees or by the method I mentioned before. Don´t try to melee a troll at this early stage because he will stomp you into the ground. Use a bow.

If you have unlocked the special ability from the deer boss eiktyr use it to reduce stamina drain while you deal with the troll.