Truthfully I think the writers are purposely doing this about Corphish.
There has yet to be a single episode where Corphish goes to Neverland like Bulbasaur in the mysterious garden episode.
Yet Corphish still won't stinking grow up. I'm sure it would be way past its evolution level if levels exist in the anime but something is just stopping it from simply gaining a few pounds and a star on its head (A Crawdaunt really is just a fatter Corphish with a star on its head.)
At this point, however, I'm thinking about giving up wanting Corphish to evolve because it has been with the show for so long, continues to miss the opportunities and, somehow, manages to kick a** (fat a**'s for that matter

). While there isn't an episode where Corphish goes to Neverland, I might as well give in and say its joined Bulbasaur, Squirtle, Psyduck and others in the island where you can fly and you never grow up. Welcome to Neverland Corphish
As for the episode itself I knew Ash would win. I hope I have the patience to watch this episode because the battle sounds great. With Tauros battling and, of course, the rat saving the day, this is a typical battle episode but still sounds good.
And also, I think it was Eddiursa in another thread that the Battle Pyramid was near Pewter City which blows off my own theory about the gang going the long way around to get back to Pallet Town or wherever, those game producers better hurry it up with D/P or were in for another filler League.