Well Warcraft doesn't really do "energy weaknesses", exactly. Undead canonly get burned more by the Light but it's because their Undead state is being.... seared constantly by the magic? Like it's not a "hnnnn turn away from holy energy" thing, and they don't take extra damage in game its just flavour, which is why it's really up to you! He only has one Undead specific spell which is "shackle Undead," which is easy to have not work!