Ash-Greninja is fine where it is.
Keep in mind that S rank is on the same level as Tapu Lele and Landorus-T.
Ash-Greninja is walled by quite a few mons. Even with Specs it can't lay a finger on the likes of Fini,AV Muk, AV Magearna,etc. And yes, I'm applying this to rain too.
Rohon-W can...