User blog comment:B214/High School DxD New Episode 3 Discussion/@comment-16075832-20130723034745/@comment-1359322-20130723084044

Actually that part is Anime only, it didn't happen in the canon story. Also you won't be able to see it yet but Issei will fully master his Balance Breaker later, and futher evolve it. Also if you follow the LN, Issei is right now the most powerful member of the Gremory Team.