I know it can't block HP / MP drain attacks for sure, I had those bypass DS all night long when fighting Zak.
I'll have to test this more on a much higher variety of monsters to see if this is truly the case.
EDIT: so I just tested this a couple times on all of the Time mobs in Ludi [Warped / Forbidden time maps], without a shield, and majority of the magic attacks went through DS, but there were some cases where DS actually proc'd and blocked / guarded against some of the magic attacks. Notably Spirit Vikings, Ghost Pirates, Death Teddies, Soul Teddies, and Master Soul Teddies were all "guarded" at least 2+ times.
Maybe it CAN block magic attacks, but the chance is extremely low?
I'll have to test this more on a much higher variety of monsters to see if this is truly the case.
EDIT: so I just tested this a couple times on all of the Time mobs in Ludi [Warped / Forbidden time maps], without a shield, and majority of the magic attacks went through DS, but there were some cases where DS actually proc'd and blocked / guarded against some of the magic attacks. Notably Spirit Vikings, Ghost Pirates, Death Teddies, Soul Teddies, and Master Soul Teddies were all "guarded" at least 2+ times.
Maybe it CAN block magic attacks, but the chance is extremely low?