Adept of the Wilds:
While in any wilderness terrain, wood elf healers can imbue additional natural protections into the sanctuaries they surround themselves in to provide them extra protection against any metal based weaponry. (thinking like a druid's protection from metals- or maybe just Iron if it's too strong otherwise.)

- Would only apply to their own sanctuary, not one's given to others. Might pass through a bond though for nexus.