Ehhm sorry for interfering, but on this topic, I have my own thoughts. Here in this comment, I made one suggestion:
Imagine that your real "body" was dropped into Abyss. So none damage is working, because real you is inside other dimension, like Obito from Naruto. Your Abyss body could stop temporaly to regenerate, because you cut it into pieces, but it's still nothing else then Abyss. We saw how original Abyss body looked like when Sabo saved Kobra - some dark shadows, or entities. So you need to break into Abyss to kill the immortals. Zoro was the only one who already break into the Realm of Dead. And I'm not sure if the Abyss is the same realm.
I disagree that you need to Kill Imu, but you need to break into the Abyss.