Invincible in the Apocalypse from the Start

Apocalypse Novel Where the Protagonist Is Invincible from the Start, with Supreme Power and No Harem

I want a novel where the male lead becomes invincible and nobody can beat him, like that apocalyptic novel I read where he used a cheat tool and was completely invincible. The hero should be really cold-blooded too.

Infinite Supplies + Pure Apocalypse + Unlimited Firearms = Invincible

Any Apocalypse Novels About Transmigrating into a Book?

I want a male and female lead, and neither of them should be a self-sacrificing saint.

Apocalypse Novels with an Overpowered System