Hi Worstgen,
Just thought I'd like to point something out.
King is like the last known descendant of the God Kingdom. Supposedly, they all had that same resilience and many of the ancient ones probably had amazing Haki too.
So, if King is so hard to beat, then why is everyone except him dead?
It is truly a mystery that Oda hasn't revealed yet. I believe he will in the future. But in the meantime, here is my speculation:
Lunarians are persecuted by the World Govenment, and we know they lived on the Red Line way before Marie Geoise existed. This means they were probably enemies of the World Govenment in the void century, and fought against it. They were considered gods because they are really hard to kill, but I think the 5 elders or at least Im knew the secret of how to kill them. They probably had a weak point, like Achiles had his heel or something. It could be that this weak point is related to when their fire is quenched, that's why it unnerved King so much when Zoro found out about it, but it doesn't have to be the whole story.
It could be that most Lunarians were killed in the void century war, and those few who were left were killed as they were caught by the Govenment throughout the 800 years since that war.
I believe King was the last one who was left because he had a purpose to fulfill. It could even be that others died protecting him so he could live on until he could meet Joy Boy. The legend of Joy Boy was part of King's Lunarian upbringing. He was the one who told of it to Kaido. Only Joy Boy's nakama and their descendants knew the name "Joy Boy". Others knew him by the name "Nika". So I think King is a descendant of one of Joy Boy's nakama and that's why he had to live no matter what.