Okay how about this Undeath's Eternal Foe?

[BoredGM]

You know what's always bothered me a bit? The apparent uselessness of turn undead with monsters of the same or slighlty higher hd. Whatever hd means right?

How about this?
1.) We set the duration to default spell Undeath's Eternal Foe to an hour/level.
2.) We make it so that while the duration is in effect, the cleric can use turn undead and destroy undead at their hd and perhaps slightly higher.

This way, the cleric would be the best at dealing with undead, since they can just use the spell and go "pew pew pew" and hordes of undead are down in PvE.

[Apokriphos]

Description: All allies in the area of effect will receive the following bonuses: immunity to negative energy damage, immunity to level/energy drain, immunity to ability score decreases, immunity to poisons, immunity to diseases and a +4 deflection bonus to AC.

Unbreachable.

I think it is currently an extremely powerful spell, and do not think it should be turns/level. If anything, it should be breachable.

However, I like the second suggestion/idea. A small boost to turn level ability would definitely be thematic for the spell.