User blog comment:B214/Question and Answers 3/@comment-4547267-20140629150343/@comment-1359322-20140701002157

That's the issue, Issei himself doubts that he can beat Sairaorg is they do fight again, that's how close their level of strength are. But since i'm positioning it based from Volume 17, where Issei had gain the Longinus Smasher, i just placed Issei as stronger than Sairaorg.