Get through to Darkness.

"Aqua might be able to exorcise him, but Darkness's stats would make that harder, even if she could hit him!" Megumin mused. "We could also just defeat Darkness and destroy the mask, but that doesn't seem plausible at this rate either! Maybe if we could stop her movements, I could hit her with Explosion, since we know she can survive it, but both options require hitting her in the first place!"

"We could also try getting the mask off for Aqua to exorcise, even if he could move about like that, he wouldn't have access to Darkness's stats anymore, and be an easier target," Xylixia mused.

Kazuma then had an idea. "...Darkness! Fight back against Vanir!" he roared, taking the only chance at pulling this off. "Are you really so easy any random Devil can control you like that!?" He was counting on a combination of Darkness's willpower, her previous ability to fight Vanir's possession, and maybe his control of her through the Sex Note.

To his amazement, it seemed to work as Darkness came to an abrupt stop. "What!?" she demanded, whipping around, glaring at Kazuma. "Who's easy!? I was just letting him control me! I can take this mask off any time I want!" she snarled, reaching up, and ripping off the seal.

"Gah! Incredible!" Vanir grunted. "Getting the tag off, and resisting me, even now! But after all the trouble I went through to control you, and with how powerful you are, I'm not giving up this body so easily!"

For a moment, the two struggled as Darkness tried to take the mask off, but was unable to with Vanir fighting back. But she was clearly able to keep him from moving. "Megumin! Blast away!" he shouted.

"Alright, ready!" she shouted, charging her blast up.

"Is this quite safe!?" Sena demanded. "You're going to hit your comrade!"

"Lady, we've done this like a million times already, Darkness will be fine... Though she doesn't have Protect on her like usual, so she might be more banged up," Kazuma admitted.

"Just do it Megumin! I can take it!" Darkness shouted.

"Wait, what!?" Vanir protested.

"... it's your call now Vanir," Darkness told the devil. "Either separate from me and get exorcised by Aqua... Or let yourself get blown up by Megumin as I hold us in place."

Without hesitation, and realizing he was about to lose, Vanir replied, "I am a devil. No self respecting devil would ever willingly allow themselves to be exorcised."

"Okay then. For what it's worth, I did enjoy our brief time together..."

"Don't weird me out in my final moments!" Vanir protested.

"Explosion!" Megumin shouted, blasting them both without restraint.