[Vicious Love]

Questions of how to reduce fatigue aside, I've found that Wither Bones can be pretty effective against Ermorian hordes(Particularly soul gate), since accuracy is irrelevant, is always terrible against small Groups of weak undead, since your mages insist on casting the overpowered spell, but was ultimately meant to be used on elite troops, SCs and mini SCs.
As has already been mentioned, wither bones ir armor negating and cannot be resisted by magic. Cast it on a group of Knights of the Unholy Sepulchre, Shadow Vestals, Bone Fiends, Wights, or whatnot, or on a Bane Lord or some other undead SC, and watch the calcium evaporate. Or possibly liquefy. Either way, works every time.