The rule that says that "Only magical attacks have any affect upon the Slann", does that prevent Miscast damage? Nowhere I could find in the BRB says "All damage done by miscasts is magical" or something similar. Obviously the one where the Slann is removed from play still work because that's not damage, but the template damage or the auto S10 hit?
From page 7 of the FAQ: So yes, miscast damage definitely does count as magical and will hurt you even with ethereal.