User blog comment:Ixarising/High School DxD Dream Battle/@comment-71.190.146.213-20140208124820/@comment-24501441-20140208130913

And remember preparing for the fight, Rias said the were "immortal" but not invincible. You have to beat them with absolute power, like Issei did when he boosted the damage of the cross and holy water, or extend the fight so long to wear them down where they lose the fight. Ddraig said the same thing to Issei during his fight with Riser. Purification mode, as seen last volume, would definitely be an absolute power that could defeat a Phenex.