Unto The End

"Show and tell" is a two step process.
(You can use the Shiny Pendant to blind the first troll and steal the key, but you will want to defeat the Troll you can skip, for an achievement related to killing them. If you follow this method you can skip some tough fighting later on in the game.)
1. Earn the Woad Kipu. At 44:30 in the video below, sheath your sword and then give an herb to a hurt elder in exchange for the Woad Kipu.
2. Use the item to avoid combat. Just a bit further when proceeding underground, show the Woad Kipu to the men guarding the elevator (47:45 in the video below).

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.

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You can get this achievement pretty early on. If you go trade an herb for the Shiny Pendant (which will also get you the achievement Master trader), you can use that pendant against the trolls to temporarily blind them. So use it against the troll guarding the Heavy Stone key and you’ll have enough time to get a couple free hits on him or just grab the key and leave if you already have the Trollbane achievement.