Defeat the envious survivor.
This will most likely be the first thing you do after entering chapter seven. A certain student of sorts has decided it is time for the learner to replace the master, and he has kidnapped a woman to help prove it. He has quite a bit of hit points, but he is also rather easily stunned by melee attacks allowing multiple hit combos to quickly chip away at his health. You can also choose to combo a weapon during combat to distract him, but I didn't find it necessary with my follower there helping me out a bit. The achievement will pop no matter what you decide to do with him after the fight.