User blog comment:Water Dragon Slayer/DREAM BATTLE CHRONICLES: SECOND MATCH!/@comment-5552025-20130724035915

I think that Mavis is extremely powerful, but I doubt she would be able to single handedly defeat Zeref. If there were any single person who could handle Zeref, he wouldn't have been so feared or such a legendary figure in terms of strength.

Looking at the demons he's created, Lullaby was pretty deadly but Deliora was stronger than a potential wizard saint. If Ul couldn't defeat Deliora and could only seal him with her life, it would be terrifying to see what Zeref is truly capable of. He also was said to have helped Acnologia become a dragon, something we have no idea how to do.

Plus, with his recent declaration to massacre humanity, he must be much stronger than would be necessary to do so if he was willing to make such a threat. Seeing as there are 3 other saint wizards we haven't met, and to combine everyone in the world (not just Fiore) would make him need to be unimaginably powerful.