I've been able to successfully use this strategy three times on both balanced and Tactician, so sharing it here for anyone struggling against him.
This should be done with any character in your party regardless of class.
-Savescum (or get lucky) and have the nurses kill each other, text choices below:
>any of three initial choices you first get (do not choose attack, you'll have to fight him and the nurses at the same time, making it harder) should work.
>Choose any of the persuasion options (One does have the nurses kill him, "They need a better subject to practise on first. Not a student, but a master", but it does not trigger the achievement!)
>Once the cutscene of the nurses killing each other finishes, you have the option to persuade him to kill himself or attack him. Choose the latter, as I previous mentioned, the former option will void the achievement.
As for the strategy:
I have two characters on him doing damage, and another taking out whatever nurse he resurrects. If you have ShadowHeart or another healing character, I recommend keeping them station above where he can't target them, but can easily heal and provide some damage doing the fight.
This should be a simple and effective method to prevent him acquiring any tools from his nurses to do lot of damage and grant you the achievement.
Good luck hunting, gamers!