I think Imu & his Commanders are Opposite of other Living Beings,
Meaning instead of Physical Form that Controls their Spiritual Energy (Haki), Imu have a Spiritual Form that Controls his Physical Energy,
This basically changes & reverses everything We've seen about Haki,
When Normal People use Haki, it allows them to Interact with Other's Haki (Sense, Touch, Conquer),
This means that when Imu use his Physical Energy, it allows him to Interact with Other's Bodies in many ways,
The Solution for Imu would be Opposite/Reversed Concept of CoA or CoC,
Meaning you need Physical Energy/Form "Created" from Spiritual Energy, not Opposite (Spiritual Energy Created from Physical)
For Everyone (Including these Six who Attacked Imu), their Bodies Command their Spirit/Haki,
But what you need instead is a Spirit/Haki that Commands your Body,
We've seen such Character who was able to Modify his Body using his Spirit (Opposite of how Haki works) & it's Zoro with Asura,
And I don't think it's a Coincidence that Xebec's Son turned out to have a Unique/Special Body & also have an Asura Jolly Roger,
If Nika Existed & Sniper King Existed & The Devil Existed & Ancient Robot Existed & Sora/Stealth Black Existed ... etc
Then Asura also Existed, and don't forget that in Buddhism, Asuras are known as "Enemies of Gods",
I think Key to Hurting Imu is turning into an Asura,
And not all Asuras have Multiple Heads or Multiple Hands, that's only Zoro's Form,
Maybe Nika is also an Asura, maybe Mythical Zoans are based on Asuras,
Maybe that's why lot of Imu's Followers have Mythical Forms (We've seen Seven so far & still counting),
It's possible that They are those who fell against Imu in the Past,