User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170414184220

I just thought of a creative way that Issei can use his Penetrate ability. Now we know that devils are weak against holy attacks. Yet the more demonic power you have, the more resistance you have against holy attacks(well it depends on who it comes from). So Xenovia's Durandal Cannon wouldn't work as well on Diehauser Belial as it would a mid-class devil. Because his demonic powers would acr as a barrier/shield against her attack. Not that he wouldn't take damage, but it really wouldn't finish him off. Now that's where Issei comes in. He could transfer his Penetrate ability to her holy swords so she would be able to penetrate through Diehauser's demonic powers that act as a shield against holy attacks.

And also Issei could probably use Penetrate to break through Sairaorg's layer of touki covering his body. That ability really has a lot of uses.