I really doubt you can block them or at least not the rain or the one where they sorround you,, you might be able to block the frontal dark magic spam but I'm sure it will break your guard before you can block them all.
As for shield the one with the highest magic resistance is the crystal ring shield, followed by the crest shield and if you can use magic magic shield magic grants a massive amount of resistance for a very short period of time, I have been able to block lake ass hydra watter attacks with it.
I'm pretty sure the Ring Shield is better against magic (but pretty weak against physical, and Manus' physical attacks are more dangerous than his magic, imo). The problem is that dark magic breaks your guard so well. The best thing you can use is the silver pendant. It is pretty easy to find (though you have to have a light source, ie. maggot, lantern, or spell). You use the SP like any other item but it is infinitely reusable and it deflects all dark magic. If you're having trouble with his magic attacks, this will be essential.
Repugnant is the creature who would squander the ability to lift an eye to heaven, conscious of his fleeting time here.--TOOL, "Right in Two"