This is a MISSABLE trophy.
This is done during the first mission of the main story after making it to the Teeth of Naros. You'll have to battle a boss and kill him before killing the other two enemies that fight with him. For more details, click the spoiler.
*** Spoiler - click to reveal ***During the main story Rites of Passage you will have to kill Warlord Kahrunk. If you do so without killing his two kobold attendants, who will heal him throughout the battle, you will get this trophy.
The easiest way do do this is to make sure you have the ability to enter Reckoning mode before you enter Kahrunk’s Tower Peak, and then wait for him to run over to you (leaving the kobolds by the ledge so they don't get hit by any splash damage attacks). Enter your Reckoning mode (+) and attack just him quickly until the option appears on him. Select it, and then it will be safe to kill the attendants. Trophy will pop either while killing the attendants or just after.
DO NOT kill the attendants (knock them out) in Reckoning mode as well or you will not get this trophy. If you do, you will need to let the Reckoning mode end without selecting the kill (time out) and then kill Kahrunk normally first. Safe bet is to put a save just before entering the tower, or the tower peak so you can retry if needed.