It's been awhile since I played, but I'm pretty sure that you have to sneak up behind him.
"*Defeat Optimum Alfred
Wear your rat suit. Go to the sun deck. He is at the opposite side. Do
not go near him, he will hit you. Go scare him, ratarang him and kick
him. We will fly off. Nearby should be 4 more robots. Go take care of all
of them. Now Alfred should be back. Go fight him. He will run off and
there will be four more robots. Beat them. Then Alfred should be back.
Fight him and that's it. This fight isn't hard at all. I was surrounded
by 3 of the robots and they cornered me and the camera was real close. It
looked scary as hell. Three smiling robots kicking the crap out of you at
night."
-GameFaqs.com