User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-23847729-20140622210010/@comment-24866657-20140623233350

You have to include things like speed, reaction and willpower, too. Simply having an ignore defense attack doesn't mean the enemy would stand there tank it, he could dodge it or intercept with another range attack.

For now Issei still not on the same level as Sirzechs, I would be upset if he's that strong already, but Ishibumi did a good job using dragon-slayer users to pin Issei down. I think Issei more or less is on Tannin's level.