This achievement unlocks in normal game play when you kill the 25th Niskaru. Niskaru are present in significant numbers in the main story as well as faction quests and side quests.
Any Niskaru killed counts towards the achievement. There are various types—Niskaru Hunter, Niskaru Bloodhunter, Niskaru Tyrant, & Niskaru Horrinox. A number of bosses during the game are 'named' Niskaru: Azaghaal
—The Great General
main story quest, Fyragnos
—Worth Fighting For
side quest in The Red Marches, Enfamanir
—Shadow of Enfamanir
House of Ballads faction quest, and Balor
—Breaking The Siege
main story quest.
The Warsworn faction quest has, perhaps, the greatest concentration of Niskaru. However, one would unlock the achievement just by playing through the main story. The 30 second video below shows the achievement popping after the 25th kill during a Warsworn faction quest, The Road Patrol