User blog comment:AYET/Dragon slaying business!/@comment-3119203-20120209133747

Panda has a point, the Ryuma that Zoro fought was not the real Ryuma: it was Brook's shadow in a powerful body. So the body was limited to Brook's moves and techniques. If Ryuma had actually been alive, it would have been a much harder fight.

You say that Zoro "only" needed one sword. Pre-timeskip, one of Zoro's strongest attacks was Ittoryu: Shishi Sonson, the only move he had capable of cutting steel for a while. So, to be honest, I don't have a problem with Zoro taking out the dragon. What I do have a problem with is Luffy being not as strong as Zoro, but that is a story for another day.