Proving your point with evidence is one thing, throwing an insult in there is another. Please don't.
Separate from the admin order there, while I don't doubt holy water exists in some form in the Nasuverse, I don't know if it'd have any effect on Angra. While Angra is All the Evils of the World, I don't know if things powered by Christian belief specifically would have any particular effect for certain. Angra Manyu has Persian origins, after all. Plus even if holy water does have an effect on Angra, I doubt it would cause significant enough damage regardless.
Let's take a look at Avenger to compare- he was basically unkillable because of the timeloop, and he had one hell of a counterattack, but in the end, he was still the weakest Servant. Bazett had to do most of the fighting, and I doubt he would have gotten as far as he did without her and her usage of Fragarach. I doubt he'd be able to manifest and maintain an entire castle, let alone do so even as an organ while maintaining a separate physical form.
Also, the castle is, as Magos has just proven, very large. Even if you ignore what Nachos pointed out and apply her weakness just as is, it would take a whole hell of a lot of effort to destroy that castle, basically rendering her unkillable in addition to being quite powerful.
...Though, again, Elf's call what's cool or not RP-wise. This is all just my opinion. Also, behave in here (this means everyone, by the way).