"Element Guy? No Respect, I Tells Ya" in Brave and the Bold (vol. 1) #68, by Bob Haney (writer), Mike Sekowsky (penciler), Mike Esposito (inker), Stan Starkman (letterer), and colorist unknown
The only four issues of the original Brave and the Bold series I have are all in the Showcase edition of Metamorpho. This one is about as representative of Bob Haney's brand of lunacy as any. Riddler, Penguin, and Joker team-up, and keeping trying to dose Batman with weird substances. The Joker pulls it off by booby-trapping the anti-booby-trapping switch in the Batmobile (seems like a design flaw by Wayne Enterprises there), and it transforms Batman into the thing seen above, temporarily.
He appraoches Metamorpho to keep him under control if he changes again until a cure can be found. Except Metamorpho fails miserably, then fails again to capture him later (by which point Bat-Hulk has press-ganged his three enemies into committing crimes with him.) He's only stopped when he grabs a TV antenna, intending to smash Rex Mason with it, and it gets struck by lightning, somehow curing him, even though electricity was not involved in the transformation at all.
Two things: One, I'm not sure Bob Haney's stories operate by any internal consistency, let alone any other kind. Unless "the universe is a weird place, man" qualifies. I know, this is not news to anyone who's spent any time on comic blogs in the last 15 years. Two, I'm a little disappointed Grant Morrison didn't bring back this version of Batman as Batsy's secret, secret identity in Batman R.I.P., instead of the patchwork Batman of Zur-En-Arrh.
The other three issues are Rex Mason's first two appearances, his origin and "The Junkyard of Doom", plus what's really more of a showdown with the Metal Men as opposed to a team-up. Although that one is notable for Will Magnus somehow curing Metamorpho, but Rex having to naturally undo the change to save the Metal Men who are under outside control, plus his groovy lady Sapphire (and also her scumbag dad and his caveman lackey).