I doubt he wanted to defeat it. He proclaimed all religions legally equal and didn't forcibly impose one on his people, much less feed Christians to lions. His own ascetic brand of Paganism alienated him from traditional Pagans and probably drew others closer to Christianity.
It depends what you think he was trying to achieve. Extirpate Christianity, certainly not. Create a new and vigorous paganism, dubious. But his "divide and rule" tactics had a chance of fragmenting Christianity, and perhaps preventing Christians from being dominant any longer.