User blog comment:B214/Shin DxD 1 discussion/@comment-39.51.215.62-20180722023923/@comment-28884330-20180722075809

Crom has only surpassed Ddraig in base specs, this doesn't account for special abilities. And even with that, Vali in DxD L, who is Heavenly Dragon-class, could fight evenly with Crom so the amount by what Crom has surpassed Ddraig and Albion is not that substantial.

Issei in his Pseudo-DxD is only slightly weaker than Vali, and he has two Dragon Slayers in Ascalon and Ascalon II, so if anything he is much better suited than Vali to fight Crom due to them, and since Vali could fight on par with Crom, Issei should be able to beat him. No need for True-DxD.