Hey Blood,
i think you are refering to "Diamond of the Indomitable" which stucks only in weapons and has the ability to proc twice per minute and gives 4305 armour..In our case (demo boss) i dont think that will help a lot as we need fire protection or immunity (Immunity Rating – giving you the chance to fully resist magical attacks) in general..
Last but not least, i dont know what kind of protection demo boss has (fm melee/range attacks or magical attacks) but generally its nice for melee chars to have combat rating for both (a) 1HE/1HB/2ΗΕ/2ΗΒ (+dmg against armor) and (b) combat rating (fire) or (cold) etc (which is used against opponent's fire or cold etc protection..)
P.S. I dont know whether the fact that demo boss does fire dmg means that is less vulnerable to cold dmg