MagisterCultuum
Great Sage
Life II "Destroy Undead" totally destroys stacks of Stygian guards, and since you already have mages, it shouldn't be too hard to secure a source of Life mana to learn it.
Destroy Undead is useless against Stygian Guards, unless perhaps if said Stygians were upgraded fro drowns (although I think even that has been fixed). Stygians are demons, and the spell only works against the undead.