Trollbane is earned after killing both the trolls. Both trolls are found along the story path so you do not need to worry about seeking them out.
The first troll can be found at the 24:00 mark in the video below.
-The way that I defeated the first troll would be to attack with
and then roll away twice to avoid his incoming attack. Just rinse and repeat until he is defeated. You can also just follow the pattern shown in the video to duck his normal 2x attacks and then dodge his sweeping overhead strike.
The second troll fight can be found at the 35:00 mark in the video below.
-The second troll is trickier, he swings around a dead deer that will insta-kill you. Make sure to duck this attack or have rolled out of the way to avoid contact with it. The second was a simple formula for me as well, blocking low to deflect his spear and then ducking when he swung with the deer.
If you are having trouble, you can temporarily blind a troll by using the Shiny Pendant object.
For all achievements in the game, you can follow this awesome guide on YouTube:
All credit to Trophy Tom. Please like or subscribe to support him. Please note how he has conveniently labeled sections of his video based on which achievement is next or what item you are tracking down, this can be seen if you watch his video on YouTube on the video progress bar instead of the imbedded video here on True Achievements. It is a very well done 100% guide.
*If this solution helped you in some way, please vote positively. If you think it needs improvement, please let me know how in a private message or by leaving a comment.*