I don't know. Plate Armor is probably still his best prep. Without being able to worship gods, you can't exactly stack a lot of resist.
Presuming you're a level 9 character, plate is worth 18 DR. For the dragonshield to be better, the boss has to deal more than 100 damage. Unless we're talking Halls of Steel, I think the dragonshield is the better of the two here.
As an added twist, you can find the plate armor in a dungeon shop, but the dragonshield cannot be found in this manner.
Also, I don't know if it is a bug, or intended, but if I have, let,s say, 150/100 HP and that I convert a life pendant, my max HP does go down by 10, but my current HP becomes my MAX HP. So I'm losing all the life steal I did.
Any time your max HP changes, you lose any overheal you had. This includes leveling up. It's actually possible to lose HP by gaining a level if you are significantly overhealed.
Only exception I'm aware of are health boosters.