With some light testing, this shield does NOT reduce the damage of Moonlight GS. Whether or not Moonlight buffs miracle damage and if this shield affects that I am not sure.
Correct me if I'ma wrong, but if I have Catalyst in left hand, Shield doesnt affect my spellcasting since it's swapped off? At least Magic Defence Buff icon disapears when it's on swap-hand.
That's correct if you have it equipped but do not have it pulled out it doesn't affect your spell casting. I use mine when fighting a spell caster so I can block his spells while I run up and stab it. Then I put it away and go back to using magic.