If you are working on every trophy for this game, than this one will come naturally as you make your way through the Warsworn questline. Throughout the questline you will be faced with nearly 40 Niskaru so don't feel you need to grind this one out to make it count. Also, once you get to Klurikon a lot of Niskaru are randomly spawned enemies within the world so again, simply playing the game and working on other trophies will net this one no problem.
For those looking to grab this particular one, here's a helpful tip found on TrueAchievements to help. 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" which sends you south to a 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.