User blog comment:B214/Volume 23 discussion blog/@comment-172.56.16.22-20170422032237/@comment-29963364-20170422051031

Even in his BB armour, Issei out-classes Rias in all aspects, except for Demonic Power and Magic.


 * Speed: Issei > Rias


 * Strength: Issei > Rias


 * Power: Issei (hand-to-hand combat) </= Rias (demonic power)

If Issei can use his speed to get within punching/kicking distance from Rias, she won't be able to use her PoD to full effect, and Issei can one-shot her.

Rias can keep her distance from Issei, she can user her PoD to (possibly) over-whelm him. Her Ultimate-Class power makes her PoD capable of easily destroying Issei's armour, and another hit would fatally wound Issei. So, theoretically, Rias can 2-shot Issei.

But, Issei is exponentially faster than Rias, even in his BB armour, so he can easily dodge her attacks. So the only way Rias can effectively attack Issei is with a massive AoE attack, but that would exhaust her Demonic Power too quickly, and even if she destroys Issei's armour with the attack, Issei can instantly re-materialize it, zoom up to Rias before she can react, and ealisy 1-shot her with a punch and/or kick.