"Paraphrasing Nep:
Saving against certain AP spells depends marginally on the smarts of the AP, with the smarter APs having more success than the dumbasses.
Spells affected: magic missile, chill touch, lightning bolt, fireball, deafen, worldbind and iceball
Quoteth the Zulg.
'force of will' has a noticable change against anti-paladins.
'iron grip of azazel' has a noticable change for some anti-paladins."
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forums.qhcf.net]
I remember they did something similar for Paladins with Wrath, but can't find any info on it.
For most stuff, it is the level of the caster compared to the level of the target and their saves/resistances. Spellcraft gives the caster a higher effective level for that spell if it gets spellcraft. The reason the Imps played human shamans was most likely for the PK range, especially more lower leveled enemies.
- Paul