What in the heck is a reverse bullet hell? If a bullet hell is "so many bullets" then I have to image it's like... Not many bullets at all. That sounds too easy.
Normal bullet hell (Touhou): the enemies shoot a hell of bullets at you. And your job is to avoid it.
Reverse bullet hell (Vampire Survivors): you shoot a hell of bullets at the enemies. And your job is to shoot even more, bigger, stronger bullets.