Isn't there one where Rachel herself says something about "morph into an elephant and STOMP YOU FLAT" ?
You might be thinking of Jake in MM3, the Agincourt scene:
"So, there we were, suddenly appearing in the middle of a bunch of tents full of guys wearing armor," I said conversationally. "Naturally we figured we'd better lie low. Not attract attention. Not cause any trouble."
<Are you really mad?> Rachel asked.
(Jake and Marco go to pull the poor knight out of the mud)
"I figured I'd try the subtle approach," I said. "But, of course, that's just me. It hadn't occurred to me that what I should do is morph into elephant and STOMP PEOPLE INTO THE MUD!"
<You are mad.>
"Why would I be mad? Just because at the very moment I'm thinking 'Cool, we snuck past the guards,' I suddenly hear and ELEPHANT?"
Such a fantastic exchange. But now that I'm rethinking, I think you might be right. I think Rachel might have said something along those lines to David.