User blog comment:B214/Volume 23 discussion blog/@comment-77.188.244.180-20170422171209/@comment-31137559-20170422180031

Again a weapon is only as powerful as its wielder. Crom Cruach is Heavenly Dragon level. It's not that he's immune to it but he's too strong. Xenovia has Durandal and Excalibur. Does that means she could take down the likes of Sirzechs and Ajuka because they are devils and are weak against holy power? Vali received a curse from the Ultimate Dragon Slayer and it didn't even kill him. It depends on the power of the individual. And Issei did deal damage to Grendel using Ascalon. Grendel was enhanced by the Holy Grail upon being revived by it. And Issei still dealt lots of damage to Grendel using Ascalon.

Issei isn't a swordsman. He will use Ascalon when he's fighting dragons but he's a hand-to-hand fighter. As you've realized all the dragons they have been facing aren't normal. And Issei has been learning how to use it. But look at the dragons he has been facing. He can channel Ascalon's aura into his punches and kicks. Just because you don't see him training to use Ascalon doesn't mean he isn't. Issei and his friends train offscreen. Or do you need to see every training session of Issei to prove he's getting stronger?