User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170318001642/@comment-29963364-20170318010147

Let's not forget that when Ishibumi first listed the Top 10 Strongest Beings.

"In no particular order: Ophis, Shiva, Vishnu, Brahma, Sakra, Thor, Typhon (or Fenrir), Hades, Aten, and Lugh."

Notice the "Typhon (or Fenrir)." That implies that Typhon and Fenrir could be considered tied for that place on the list, meaning that they are about equal in strenght. And Fenrir is said to be on par with a Heavenly Dragon (before Vali and Arthur nerfed him).

Therefore, Typhon is Heavenly Dragon-Class. And considering Issei beat Apophis, who is also Heavenly Dragon-Class, it's not unreasonable for Issei to (at least) fight evenly with Typhon.