User blog comment:B214/Shin DxD 1 discussion/@comment-1359322-20180721111033/@comment-28884330-20180721174736

ExE is most likely a power-up that won't happen in the DxD series itself. Even Ajuka said that it's not the time for it.

Aside from that, if you think that power is the only thing that matters in Rating Games then Diehauser should have been unable to beat a single god-class team. And yet he beat not just one, but several.

I don't think Rias will be the one to defeat Issei, but it would only be logical that he loses at some point in the tournament, likely the semi-finals with the hinting at what he requires to be Maou.