I should note that August is not comparable to Rufus. I give August the benefit of being able to cast dragon slaying magic, because he uses copy magic and he managed to use a fire spell against Natsu. The spell is also would have been as powerful as Natsu's attacks, even if it wasn't dragon slaying magic. Rufus was never shown using dragon slaying magic. He used a spell that he's already used before. Even if Rufus could theoretically conjure up dragon slaying magic, he still wouldn't be as strong as Natsu. Rufus is still limited by his vastly inferior magical power, which is something that August does not suffer from.