In "The Wolds" territory in the Plains of Erathell there is a woman named Singing Nell that lives in a tent just outside of Ayten (east side) When you talk to her she will give you the quest "The Erathell's Blessing" send you south to Windstone circle to activate the stones and receive a blessing. If the stones are activated in any order other than ascending from lowest to highest note, a Niskaru hunter will spawn in the center of the circle. While you won't receive any experience points, or any good loot from them, they do count towards the achievement, so just run in a circle activating the stones, kill the Niskaru and repeat.
**Note: Once you receive the blessing, as far as I am aware, you won't be able to activate the stones anymore other than to receive the blessing again, so get the achievement before doing so.**
Thanks to EL HORSE for reminding me of the quests name.