Is there a way to battle May again after the Delta Episode?
You battle May for the last time after credits. However, you can battle alongside her in Multi Battles at the Battle Maison
You can battle May once after the credits, on Route 103. (Source) Once you have defeated her, however, you cannot battle her again.
After you beat the elite four no, she's out of the game battle-wise. I think you can talk to her in her house , but not fight her.