Well I've played Ermor and been on the other end of some serious Wither Bones-ing, and I'm here to tell you that it's a nightmare for the receiver.

Given that undead normall come in Huge Hordes(TM), accuracy doesn't tend to be an issue; you're always going to hit <i>someting</i>. But, like priests and Banish, you're going to need a critical mass of casters to do any good. You've got to attrite them quickly (or have a rock-solid shield of melee fighters to hide behind) before their numbers overwhelm you.

Give your casters some death gems or another death boosting item to keep the fatigue cost down and increase the number of castings. Or, if you have easier access to large numbers of priests, they may be the better way to go anyway.

And there's always the luck factor. Try it a few more times before you give up on it. I bet you see better results next time.

Mark