Shadow's boost system is easy for doing just this sort of thing. And, if you want a NPC that has 100% fire immune and 20% elec immune you can do that without having to make a separate skin for that scenario. It allows you to specify a % chance of the NPC having the bonus, and allows other bonuses like AB/AC and damage. It's a great way to give repetitive mobs variety.
http://neverwinterva...reatures-system