Keep calm, this has a simple solution.
One of you shall make an offshoot where Arceus is pronounced with a soft "c", while decrying the ones who don't as heretics.
The rest will keep pronouncing it with the hard "c", under the rule of The Pokemon Company, His representatives on earth, while treating the offshoot as something of minor importance, until tensions make it so the hard "c"ers start to hunt down the soft "c"ers in an effort to eliminate all dissention. Meanwhile, the soft "C"ers will go house by house asking people if they heard the true pronunciation of Arceus, and ask for donations for The Great Crusade.
Finally, as a result, Metalizard will be caught and nailed to a giant pokeball, and that will end the conflict until someone decides that Arceus is spelled with a "z", and everything shall start again. For that is the will of Arceus, because he likes to have fun messing with people while he drinks at the heavenly bar with God, Buddha, Allah, Yoda and probably, David Bowie.