User blog comment:Pualover/Overlord Power Level/@comment-34705125-20180224183308/@comment-45.56.46.26-20180606180845

Which raises an interesting question. Do they need the World Item just to counter the power of a World Item, or is there a particular World Item that would be particularly useful against Rubedo?

I bet she has some ultimate attack that she can spam as a result of the Caloric Stone, which is why it would be pointless to even try fighting her without a World Item to negate that attack.