Lets count the number of times the word invincible is used to hype up someone.
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Its not meant to be taken literally, there cannot be three-four invincible guys running around.
Why cannot three or four invincible peoples exist together.
In my country this kind of invincible peoples exist by numbers
They are immune from getting cut, bullet cant penetrate their body etc
This kind of peoples is actually a martial artist and they learn very high level of spiritual knowledge
And the dark secret is they are mostly learn dark knowledge/black magic.
They way to defeat them is by destroying their black magic.
In NV the one person that I can say learn some form of teaching that actually like black magic from our world is Hidan, but he gained immortality not invincibility.
Dont be afraid to have many characters that is invincible roam free in the manga, because there exist a way to defeat them
But back to my other statement regarding of black magic, I dont know where you came from, but if you a western people, its hard for you to believe it because it look to superstitious, but in east asia, this kind of think is normal.