My good friend.Despite Gekkouga having a special form change because of his bond with Satoshi, I think that Pikachu's emotional bond with Satoshi is stronger, so Pikachu would probably put up a better fight and probably win based on how much he cares about Satoshi. There's also Pikachu's skill to consider given his battling experience.
I said feats. The level of bonds between either be Ash-Greninja or Ash-Pikachu won't affect the outcome of the battle.
Anyway I'll respect your opinion. I don't mean to come cross as rude, I apologize if so. It is just I am only interested in feats ~