When I'm stuck for a spelling I often first seek aid of the subtitles to at least give me a basis. Thus:

I probably could've left it as is, but I guess I just figured it'd lead to possibly trying to call him "Parn-ass" which is more maybe what Alielle thinks of him sometimes than a proper pronounciation.

So the basis of "Parnasse" combined with an "o" for stronger guarentee it comes out like it does in the English dub. At least it isn't "Palnus", which I've seen used before. "Palnus". Heheheh...