I've just gotten to the Castle of Satan and I heard that there's a really large difficulty spike after this point.
I mean, I've been managing without any issues running Raphael, Angel, some miscellaneous light mobs, a Super Gold King Dragon that I lucked into, and a friend's Baal or Apollo. It's just that I have no idea what to expect from herein because I have two extra Raphaels just chilling in my monster box.