Ok, cease fire.
In all seriousness, if one of them REALLY has to die even if temporarily, Denji is in dire straits.
Let's compare.
Asa dying:
>Would be extremely shocking and emotional for the readers.
>Wouldn't matter much in-universe because no one really cares about Asa, her death would be a footnote for Denji compared to Nayuta's.
>Doesn't advance the antagonist's agendas.
>Defuses the plot anticlimatically.
>Can't happen before her dream.
>Repetitive, makes her suicide attempts redundant.
Denji dying:
>Would be extremely shocking and emotional for the readers (no, this place celebrating doesn't count).
>Would utterly mindrape Asa.
>Would mindrape Yoru even.
>Advances the antagonist's agendas
>Greatly escalates the stakes.
>Justifies Asa's dream.
>Foreshadowed through the entire story.
>Denji himself is half-complacent with being weaponized and already accepted that he's getting fucked again.
Nevermind the fact that Denji is in the lion's den of a very manipulative devil, is acting as live bait for Asaru and already got grievously wounded (probably). There's a real chance that Denji won't live to see the end of this volume, even if he comes back later.