User blog comment:B214/Question and Answers 6/@comment-26926813-20171225175417/@comment-24595619-20171225191248

Issei couldn't fight against Apophis even with Diabolos Dragon God. The only reason that he could have killed him was the of the combined attack of both Infinity Blaster and Longinus Smasher. It should be at least at the level of a Super Devil or at least bellow it.