I tested them on Steam.
Nivus's normal ranged attack did zero harm to Saurian Blazefang and his Magic Missiles (tier 3) dealt 49 damage to the enemy.
Bruxa's normal ranged attack dealt 5-15 damage to Blazefang and her Laughing Skull dealt mostly 3 (rarely 10) damage each.
Her Skull Sacrifice did 54 true damage to the enemy. And her Voodoo Doll also dealt true damage to them.
http://kingdomrushtd.wikia.com/wiki/Nivus
http://kingdomrushtd.wikia.com/wiki/Bruxa
It seems like Blazefang's magic resistence is slight above 70%.