I've used an specific enemy character to great effect. S'pose he could serve as NPC as well. My intent was to make a character that would be versatile, a challenge, yet not immediately fatal to a group of level 2 adventurers.
Make a character that's a level 4 Cleric and level 3 Illusionist. Making him simply a level 7 Illusionist would probably make him much more deadly...
His Domains are Travel and Trickery. His Feats include Improved Initiative and Brew Potion.
Since he can brew potions and scribe scrolls, and there's really no reason why he should not be well-equipped with either. I gave him Potions of Invisibility, Mirror Image and Cure Moderate Wounds, and Scrolls of Glitterdust, Web and Summon Monster II.
The way this guy fights is casting his area disabling spells (Web and Glitterdust) and kill people that are disabled. Disabled people can be killed with a single blow (Coup de Grace). With the Travel Domain power, he can walk through his Webs as if they're not there. Summon Monster provides a couple of expendable Dire Rats that can cheaply deliver a couple of death blows on disabled characters for you.
If, at any time, he gets hurt, have him drink a Potion of Invisibility, run around a bit, drink healing potions and finally a potion of Mirror Image. None of these break invisibility.
Cast Shield at the start of battle; it lasts for an insanely long time and gives a huge bonus. Have him wear a Chain Shirt to give him really good AC while suffering only minor spell failure.
This battle lasted for hours (real time, of course), during which he would repeatedly "time out" with Invisibility, heal back up and go at it again. In the end, he turned invisible and ran away.
