When I was trying to make something like a spriggan from The Elder Scrolls, I noticed that an advanced kodama pretty much has all the abilities and power that you neeed. (I can post the stats if aynone wants them)
So instead of comming up with something entirely new for the bigger spirits, like the god of a mountain or a large lake, converting the already existing Jinushigami seems like a much better idea. Now bumping up a CR 5 creature by 2 HD to make it CR 6 is easy. But downgrading a CR 20 beast with 24 HD to something that works at roughly CR 15 is a bit more daunting, especially as I don't really have any experience how things in that power range actually work out.
The creature is intended for a low optimization P10 game and still be threatened by a large enough horde of 7th level characters. If everything they can hurl at it just bounces off without any effect, it's too strong. If it can easily kill 10 7th level character before going down, that would still be okay.
Can anyone give me any helpful advice how to approach this?