User blog comment:B214/Volume 17 spoilers./@comment-24.2.168.254-20140220012350/@comment-1359322-20140220083351

Shalba is weaker than Euclid though, plus Euclid wasn't even using all of his strength in Volume 16, and was able to overwhelm Issei, for Issei to be able to beat him in just one volume sounds a tad too convenient. Well there's also the Ishibumi was Ishibumi said about one of the theme of the fourth arc being, "brother" referring to Issei and Euclid who are both Sirzechs' brothers.