Get yourself some Quells; let's see:
1. They hate all divine casters. With a passion.
2. They are Incorporeal.
3. They can "Interdict" divine casters; basically works like a Turning attempt, but instead of turning the caster, you cut them off from all deific help (spells, turning, all that jazz) for one minute, or 24 hours if there are enough quells! You would need about 36 to Interdict a 20th level cleric though, unless your DM rules that advancing the Quell's HD would let it Interdict more powerfully.
They are my favorite undead, by the way; if my group ever lets me do the necromancy thing, I'll have at least 1 of these guys around at all times.