User blog comment:B214/Shin DxD 1 discussion/@comment-30230931-20180804194539/@comment-28884330-20180806212326

That immortality can only be beaten by someone more powerful if they have enough time to exhaust the stamina of the immortal until they cannot regenerate anymore. In Volume 2, an untrained Riser could handle Rias' attacks and continue regenerating until she ran out of demonic power. Ravel has trained, so she should have at least that amount of stamina. Even with that, there's no way Rias' team can even reach Ravel. Her team is outclassed in a 1 to 1 basis and even on top of that it's outnumbered 9 to 11, which can even turn to 8 to 13 if Rias uses Balor Form and when Ddraig and Fafnir are summoned. How would Rias' team even reach Ravel? Not to count have enough time to exhaust her stamina.