Well, spellcasters in my world know that fighters and rogues generally have weak wills, divine spellcasters have poor reflexes, and arcane spellcasters have weak stomachs. Thus, they know that the blessed, holy knights of Heironeous and Pelor are pretty much immune to any kind of magic that they can resist, well...
In other words, my bad guys are actually intelligent and behave similarily to the PCs. If you prefer to play your bad guys as idiot morons, be my guest, but everyone I play with knows never to try and charm the cleric. My players make intelligent "metagame" decisions and target their spells towards those least likely to resist: charms towards fighters or rogues, necromancies and disintegrates towards wizards, and fireballs towards everyone, but particularily wizards and clerics. I don't think it's particularily metagame to assume that heroes know how to identify who's most likely to succumb to their spell effects.
Also, this character was an extremely famous high level paladin who's pretty much a household name as a famous hero, introduces himself as "Gawain the Paladin of Heironeous," wears holy symbols like pimps wear gaudy jewelry, has a higher Charisma than his deity (assuming Heironeous doesn't have any stuff, of course) and has saves in the mid 30s, yes, my bad guys act quite appropriately.
Also, considering that the removal of negative levels pretty much hoses any kind of necromancer or anyone who wants a decent 4th lvl targeted attack spell, it's not a fair house rule.
Before you call the tactics of my bad guys "targeted BS" and "cheesy metagaming" please, molonel, learn the situation before passing judgement.