The achievement unlocks when you kill the Jottun Warchief Kahrunk
without killing his attendants. During the second quest of the Teeth of Naros
questline, Rites of Passage
; the player character must kill Kahrunk. Kahrunk has two shaman retainers who heal his wounds and defend him with a protective barrier. This is a boss fight. You are advised that the best tactics is to kill the shaman retainers first. However, if you defeat Kahrunk first, you unlock the achievement. Afterwards, the attendants will attack and it's OK to kill them.
The task is somewhat complicated by the fact that Kahrunk is a big, robust boss with a large health bar and the attendants are puny, low-level enemies. Collateral damage can easily kill them before the boss is substantially harmed. For this reason, it is prudent to use a precision weapon like a pair of daggers that targets a single target so you do not have to worry about 'splash' damage. Going into reckoning mode and using its slow motion effect also facilitates the fight.
Using a minor, greater, or master phasewalk beforehand turns you invisible and prevents the attendants from noticing you. The goal is to keep the retainers oblivious to your presence and at a safe distance. Then, it is fairly simple to keep Kahrunk isolated while you defeat him in reckoning mode as illustrated in the below video:
Note: The use of offensive potions beforehand is also beneficial. The fight is shortened and can be concluded before the attendants can react and approach the danger zone.