I just had a amazing run, which sucks cause it was on my laptop, and since I cant connect to steam on school wifi, I have basically two different versions of the game going.
playing maggy, I got Mitre as my first item, after 2nd boss I got brimstone from devil room, got a few all stats up and had a decent setup, got Krampus's head which is OP against bosses and found the "challenge up" trinket so I decided "what the hell I've got good items I can probably deal with champions" and it turns out challenge up is actually amazing for the boss rush room.
I got like 20 Spirit/Black hearts when I was already overflowing with spirit hearts.
after that I got the item that lets you shoot from behind so I just hit everything after that.
I'm currently at Shoel with the character, full spirit hearts, I think im gonna win this run probably.