I was going to suggest the send out Spivey first thing.
Also you are aware that Butterfree is doubly weak to rock types right. So it might have been slaughtered out there.
Also Onix is doubly weak to water types.
I would totally be a water type gym leader.
Hmmm, I did realize that The flying side would be weak to rock, forgot about the bug side. In fairness though, I think only Onix has a rock attack, and only one, so risky but not horribly stupid. Her speed and confusion being able to bypass the rock resistance really made her the only good back-up. But in the end, Poseidon with Bubble handled it well.
And...Why would Onix be double weak to water? It's a pure rock type, it should only have x2 vs water, not the x4 of rock/ground.
I'm actually not sure what gym type I would pick myself. Probably Steel type or water. Or normal.