In my opinion, forget about Light; a mass Stone Skin spell from the Defense branch is more than enough.

You're right in some ways. At least because you need to spend hero turns on Light spells, while Defense will always be working. But the thing is, Necromancers often develop two schools of magic, usually Summoning + Darkness. I suggest that, if the opportunity arises (when there's a Castle with Light nearby and you're offered a skill point), instead of Summoning, develop Light, and then continue developing as usual, without spending a level on the "Stone Resilience" perk. This makes the "Twilight" perk available, but it's not necessary to take it, as Necromancer's Spell Power is already good, and Light and Darkness don't depend that much on Spell Power.
I don't want you to think that I don't like Summoning (remember our argument about filler units); I just think that the most effective Summoning spell is Duplicate, and I already had access to it because I was playing as Lucretia (inherent Sorcery) and took the "Wisdom" perk.
P.S. - Mefisto, five stars!!!! I somehow didn't think of "Soldier's Luck." And yet, in the late game, I often take Vampire Lords because, in 90% of cases, my hero's first turn is "Mass Slow."