Well, I'll admit Goku isn't very bright, but when it comes to fighting, Goku is practically a genius. One thing Goku's good at is adapting to his oponents and situation, and figuring out just how good his oponent is in the middle of the fight, and figuring out any small weaknesses. Not saying that Batman can't, but just letting you know that Goku is much more than just might.
Even if he's powered down to human level then Goku's Saiyan strength would still be way stronger than Batmans and don't forget that his power would be increasing every second-hence increasing strength and speed- And Batman will have to find his head, thats flying away from his body very quickly. Will people stop with this DC or MARVEL character vs a DBZ character because whichever way you look at at it the DC or MARVEL character is getting fried unless it is one of the god-like beings then in that case it would be unfair the other way.