Unkillable Regenerador In RE4 Classif

As I said, when I was around 6 years old (18 at the moment) I remember watching my older brothers play RE4 Classic, encountering a Regenerador out of the blue in a room ( I can’t recall it ) but I swear all I remember was the shock of my brothers when this Regenerador doesn’t die even when using every single shot and gun. They ran out of patience and restarted, went to get the thermal scope, and ironically enough, this Regenerador doesn’t even have Plaga attached to it. It’s like the but in RE4 Remake when you kill the Regenerador and it doesn’t die becoming immortal. And I played classes more times than I can count and never met such Regenerador with no plaga and can’t die. + it was a PS2 game version