You unlock being able to do this in Chapter 2, as you start to be able to pick up purple flags of players where some other players anywhere in this game that has died, when you will see purple flame above the enemy's morale rank, when you kill that enemy then you avenged that player that died from it, similar like from Nioh. You can also kill the same enemy that just killed you in that area as it counts toward progress for this achievement too.