True Faith (with Turning) is a 24-point win button against undead. Or anything else subject to turning. Concentrate, win the Quick Contest, and you can easily force the undead back and push them into corners where they can be killed by your allies. Will is only 5/level, and getting bonuses isn't hard, either - +1 or +2 for good holy symbols are almost automatic for a cleric. This makes undead difficult to use effectively against a cleric with this power.
Douglas Cole has complained about his own prayer powers for his Warrior-Saint, Cadmus, who presses the win button of his anti-evil aura and the bad guys flee.
Here is another option - treat it the same way I treat fear in my games (originally griped about here.) If you fail to resist a fear-based attack, you suffer a -1 for every point you fail by for any action that isn't running away from what frightened you. That includes resistance rolls, making for a "fear spiral" if you don't leave quickly. Once you're out of the range of the fear attack (around a corner, out of sight, etc.) you can try to recover by making rolls against the original base resistance; every point you win by strips away one point of your fear penalty. Once it's zero, you're all calmed down.
In this case, True Faith (w/Turning) would cause -1 to all rolls other than those to help fleeing the cleric.
True Faith (w/Turning) (Modified)
To use this, a cleric must take a Concentrate maneuver. Presenting a holy symbol will give its usual bonus. Roll versus Will plus any bonus vs. the Will of any undead. For each point they lose the contest by, they suffer a -1 to all actions whenever they are within a number of yards equal to their margin of defeat. There are two important clarifications:
Mindless undead always run, stay outside of the protection radius of the cleric. If unable to flee, they can fight back but suffer the full penalty from their loss in the contest.
Willful undead may choose to run, or to stay within the radius of the cleric and suffer a penalty equal to their margin of failure in the contest.
Penalties last until 1d6 seconds after the cleric stops concentrating on Turning.
For example, Inquisitor Marco has Will 15, Power Investiture 3, and a high holy symbol (+2). He faces four zombies (Will 10) and a vampire (Will 16). He rolls a 12, making his roll by 15+3+2-12 = 8 points. The zombies roll 10, and lose by 8. The vampire rolls a 12, and loses by 4. The zombies are at -8 and flee. The vampire is at -4, and can either continue to act normally at -4, or attempt to flee at least 4 yards from the cleric; once there he is no longer at -4 (but will be again if the cleric closes with him, or he moves closer again).
Basically this gives powerful supernatural critters the chance to shrug off the turning and bull through, but still gives them a reason to back off from a cleric presenting a holy symbol.
You can set a ceiling, though, and say that the maximum penalty is -X (say, -10) and that after that all undead must flee.
I've tried this with fear in actual play, and it works fine (you can see it in effect here.) I haven't tried it yet with Turning, but I'm considering doing so. To balance out the reduced effect, it might be nice to add some other effect, or perhaps just reduce the cost increase from +60% to something like +40% (21 points). I haven't tried either of those. Mechanically I think it's workable, but it's setting the appropriate cost on its nuanced but reduced effects that I'm unsure of, yet.
This works better for combat-heavy games like DF, and less well for games of supernatural fear where a lowly but faithful priest might drive up a monster with a crucifix held high.