"Practically invincible" so there is some context there.
We add the fact that the Seraphim were stated to need testing in order to determine the level of attack to take with flame on, which wouldn't be the case of they were just invincible
I've been saying that for a while now.
Nobody, Hakai or even Imu's Island Nuke, can hurt Flame On.
They are simply Invulnerable.
RT Zoro was already stronger than King.
He would've beat him had he known about the trick.
CoC was to control Enma and defeat King quickly.