If I wanted to beat this dungeon boss, I would have to target it's weakness, [Lightning] magic. But since it's being using Vaniti as a catalyst for it's ongoing [Spirit Magic], even if I target it's weakness it won't be as effective.
[Spirit Magic] just reduces the effectiveness of other elemental magic, just by a staggering 20%. That 20% was usually treated as a damage reduction, and since to say that Spirit Elves themselves are resonate with [Spirit Magic], they do also get a 20% damage reduction towards any elemental magic targeted at them.
If the Dungeon Boss is as smart as I give it credit for, it would know that my [Lightning] spells won't be as effective as I need it to be.
