User blog comment:B214/Shin DxD 1 discussion/@comment-39.51.215.62-20180722023923/@comment-28884330-20180722164808

He is only slightly below Vali’s since Issei’s Pseudo-DxD is either equal or slightly inferior to Ddraig by what he said in Volume 25. Therefore, they are both Heavenly Dragon-class and so the difference is negligible. And as I explained before, unlike Vali, Issei has two Dragon Slayers in Ascalon and Ascalon II, and those serve make the difference even more insignificant and make him much better suited than Vali to fight Crom. If Crom could only fight on par with DxD L Vali, it is only logical that he will lose against an opponent who is in the same class as Vali and has two Dragon Slayers on top of that.