I really can't understand how people like you can read such a simple story and still managed to misunderstand something so simple lol. "Hey, why didn't the MC sneak attack the immortal dude and just instakill him?" yeah, he definitely would've won if he did that, it's a common cliche for dudes with hyper regeneration to die from headshots, y'know. I can name so many movies, comic books, mangas, and animes where the character who could regrow their limbs were beaten by a sneak attack and got oneshot.