Reimu B IS the definition of Shotgunning, IMO.
Both because of Duplex Barrier, but also because if your at a certain distance with her unfocused shoot and manage to hit with all the needles, you do the max damage you are capable of doing in the entire game, surpassing even that of Marisas firepower (Or so I have heard)... Anyway yeah its stupidly strong. That + her decent cherry rate is why I believe she is the best shottype overall.
Well, yes. But Youmu is particularly vulnerable - EVERY spellcard can be shotgun bombed easily.

Ten King's Retribution? She stays low on the screen anyway.
Hell God Sword? same thing. Focused damage + shotgun duplex barrier at point blank will kill her with practically no threat to you.
Obsession with the present world - everthing's fired from the sides, not from her. Point blank until she does the bullet gathering move then duplex barrier when anything gets close. If you're near a border shoot unfocused (or can do the all needles hitting) until you get a border.
Eternal Truth - just be somewhat careful about it so you don't get rammed by one of her aimed shots up close :V
Components of Kompaku: You need to be quick about it, but if you exit a wave early you can rush up to her and bomb her face. Not sure if it's viable for the first wave or not (AKA does she have super armor), I will test - it usually takes me ages to down this spellcard. :V
I've thought about a few other cards I might do it to. Notably "Law of Morality -Demon World-" which I am horrendously bad at, and Butterfly Delusion. Both normally take a while to down.
EDIT: Captured Kimontonkou. :V