So this DR is magic-based? Or is it specifically against Necromancers (as it was affecting all Necro spells)? I didn't actually use physical damage against it, so I was unaware of whether it had physical damage DR. Also I was not in a group, so I didn't see whether fire, ice, healing-based, or other spells affected it the same way as my Necro spells.
I am unsure where to look with regard to a "boss manual" to identify how I should diversify my party, or if that is simply meant to be discovered and discussed through community collaboration. I understand boss mobs are in their infancy in many ways and that these are details that may be resolved in time.