I feel like Loki's Defense is different than Kaidou's or Big Mom's,
He is definitely Tanky, but i'm saying that He doesn't have Armored Skin, He is still Normal like WB, but i have a feeling that something else is preventing him from Dying,
It's similar to when Luffy without any explanation, resisted Crocodile's Poison,
Also after MF, Hawkins said that Luffy's Chances of Dying refuse to Hit 0,
I think this is Similar Case,
In other words, Kaidou is basically Hard to Kill, but Loki "Cannot be Killed",
So if it's a Hit that can Kill him, He will ignore it,
This aligns with what Dorry said regarding Elbaph's Divine Protection,
This is what Dorry said after He was Injured by Mr. 5's Bombs:
"I am sorry... For doubting you... This is Judgment of The warrior God, Elbaf!!! If He did not grant me his Divine Protection... Then that is all!!!"
Loki & Luffy clearly have Divine Protection, i think Blackbeard too cannot be Killed, and there are few other Candidates,