OK, I know no one believes this but:

In the room in Xorian with the stairs to the left, and the transparent bridge on the second floor, there is a trick to it.

Run straight into the room to the altar. Drag everything there.

Have a cleric jump on top of the altar.

When everything is around you, the cleric does turn undead. No matter how bad your cleric is at turning, it kills it. All of it.

I've seen it done. It sounds like BS, and I thought so too. Try it.

For those that post below saying this is wrong, please comment on whether you tried it or not.

Cheers,
Ink