After finding Arhu in the deepest part of Kemm's vault he will be trapped by four spirits. In order to see them you need to use spirit vision. Once spirit vision is activated start consuming all four of them in order to set Arhu free. Arhu will keep telling you to stop because you are going to kill him but continue to consume the spirits. After all four are consumed Arhu will be free and then enemies will spawn that you need to kill. Once you kill the enemies while keeping Arhu alive the achievement will pop.