User blog comment:Trihexa/Dragon Ability - Outrage/@comment-24502975-20150606203749/@comment-26468985-20150606221613

I agree, though Rizevim couldn't easily kill them, Fafnir had the same level of power during his temporary Outrage than Rizevim, and he's the weakest Dragon King.

If Issei wanted to, he could enter Juggernaut Drive and wipe the floor with Rizevim, Sirzechs and Ajuka (of course, not all of them)'s dead corpses (because of his doing).

If Fafnir had gained the rage, he could enter Outrage and fight on par with one of them.