User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-24053170-20180716184925/@comment-27973844-20180716204848

Well if Issei would get a ability like the purgatory Flames(Damage by the Flames can not be healed) from Meliodas, than he could beat it but issei would first Need to have enough power to do so and i think the mastered unnerved Diabolos Dragon form should have enough power to fight 666.