Bui is a very powerful fighter and his axe can really deal massive damage. You don’t want to mess with this guy. That being said, he still won’t be able to take down Zeref. Zeref has enough dark energy abilities to destroy entire planets at this point. His final fight with Natsu was intense. Bui just gets overwhelmed in all areas during this fight. He won’t be able to claim the win here. Zeref wins.