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Rias and Akeno don't "beat" Mitsuru's memory, Minato learns to let her go and move on. Penthesilea only deigns to fade away after Minato assures her that he's finally ready to forge new bonds with people, instead of clinging to his past. No matter how much Rias and Akeno batter her, she refuses to stay down, and since Penthesilea is a representation of Minato's bond with Mitsuru, it says something about how powerful their connection still is.
Penthesilea's battle with Rias and Akeno is awesome on its part. And to cap it all off? The chapter closes on one final showdown: Rias and co. Best of all: Rias leads her peerage to victory, the strength of their bonds and teamwork overcoming Minato's might. Against Rias though? The Personas of SEES. The former features Personas of legendary heroes and warriors-among them King Arthur, Joan of Arc, Guan Yu, and Achilles-as befits Sairaorg's reputation as the strongest devil of his generation. In Chapter 21, Minato's challenge to Sairaorg's and Rias' peerages. Missing an arm that has yet to regenerate he goes up to the dragon and breaks every tooth on one side of its face with a swipe of his mighty shock maul. And it's not just a Mirror Match, the main cast decides to intervene once more with feeling. Between Chapters 17 and 18, Issei, in a desperate plea, borrows Minato's Persona Nidhogg to go against the DxDverse's Nidhogg himself, complete with a "The Reason You Suck" Speech. But they won in preventing Erebus' escape, and the latter two decided if they would go down, it would be laughing and giving all they had against Erebus.
All that remained were Thanatos, Metatron, and Belial against a horde of Shadows. It involved summoning all of his Personas that could ultimately stall for time for the seal to complete itself. It is always amusing to hear talk of status from those who possess so very little of it."
I am Helel, the one they call the Morning Star," it smiled at the utter, utter silence its appearance caused, "Please do continue this line of conversation.