User blog comment:FairyRaven/Fairy Tail Chapter 427: Pitched Underground Battle, Review and Commentary/@comment-76.121.234.236-20150406132532/@comment-1359322-20150407004803

@anon, maybe Natsu has mastered the Dragon Force during that 1 year gap. Since the DF has been stated to have powers comparable to a real Dragon, and Natsu did access the DF on his own when fighting Mard.