User blog comment:B214/Shin DxD 1 discussion/@comment-95.210.189.106-20180809123512/@comment-28884330-20180809195348

Strada will be kept busy by Xenovia, and Ddraig would only have to handle Balor Rias whom he can easily defeat.

Vali fought on par with Crom Cruach, and Issei is only slightly inferior to Vali's level overall. Therefore, Issei by himself has the capacity to fight Crom Cruach evenly, but unlike Vali, Issei has Ascalon and Ascalon II which are Dragon Slayers that can be further enhanced by Boost, Transfer and Penetrate. He would have a much easier time against Crom than Vali did, so it is not a stretch to say he has higher chances of winning.