I was off on the latter, but correct on the former ;)

Thanks.

I would suggest raising the HP a little, since a party of level 15 would most certainly have magic weapons that can hit incorporeal enemies. If not the HP, then perhaps the AC.
Its got more HP than most CR 15's. But this thing can disapear into the woods and be awfully hard to find, along with having a 20% miss-chance. And this is assuming the players can make the save to even approach the thing.