Turn attempts are level-independent.  They probably have four or five attempts, with a charisma of +1 or +2.  Even a level 1 Cleric can turn away large numbers of 2-HD zombies on an average attempt, and thirty level 1 Clerics can repulse a<em>lot</em>of zombies.<br/><br/>And yes, the zombies will return, but while they're alive and fleeing, they're blocking space that would otherwise be used for more zombies to pour out.  The idea is not to destroy the zombies through force of arms, but to hold them back long enough for us to pull the kill switch.  That's exactly what all that turning does: Hold back a lot of zombies for a while.