I've read various posts regarding the proper way to defeat this Lord of Shadows, Nauselom.
In my experience you MUST directly strike Nauselom with the Rod of Divinity. You can't get away with a long range lightning strike from the Rod. After you nail him with it then you can change up and hack away with whatever suits you.
It helps if you summon a couple Fiends to handle the light work while you whittle away at the boss. It really helps to have the Shield of Retribution equipped.
How to kill Nauselom
How to kill Nauselom
I am the Lord of the Waste Land; A modern day 'Man of Steel'. I gather darkness to please me
and I Command you to Kneel
and I Command you to Kneel