User blog comment:B214/Volume 23 discussion blog/@comment-78.48.65.206-20170426164843/@comment-79.25.167.4-20170427233335

yeah but not before 30 years, so of course Issei will have to train for decades to have enough strenght to be considered among the top 3-4.

Issei DxD power is still unstable and remember that he defeated Apophis before the nerfing, so right now Issei is not really at Heavenly Dragon level (especially because he can't withstand that form for long). He can reach that power only for a few moments. To be considered among the top you have to be able to stay powerful for more than 10 minutes I mean.