Monstrously, disgustingly hard. If you think Souls games push you to your limit, then give this a go and prepare to weep amidst the broken ruins of your controller.

That disclaimer aside, Super Ghouls 'n Ghosts is probably the best game in the series. I hear there's a version swimming around out there with the slowdown removed, and if that were true, then it'd definitely be the way to play it, wink wink.