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.
The problem with the Ring Shield and Manus is that he uses dark magic, and dark magic does physical and magic damage. Even if the magic rating is high it's physical defense rating is 70.0 since last time I checked. You'd probably die if you tried to block it with the Ring Shield with less than a hella lot of health. The crest shield is the best since you have 100% physical block, and 80% magic.
Dodging and the silver pendant are your best bets. --- "I disapprove of what you say, but I will defend to the death your right to say it" Need gaming help? Go here: http://www.youtube.com/kingnahum17