Considering how much magical firepower a party at 15th level should have, I think a little evil fellow trying to compete with that with magical tricks of her own would be spectacularly outdone in most cases. See Invisibility is a level 1 spell after all.
I think a solid option for an atypical BBEG is a single-class Thought Killer Vigilante.
She'd be a respected priestess to the public and an evil schemer while in the vigilante identity, and by taking the Many Guises and Everyman Social Talents, she could be the peasant, the beggar, or even the party's favorite bartender. This villain will have constant nondetection, could periodically examine the party's thoughts to see if they suspect her, and could reliably out-bluff the most paranoid Diviner Wizard.
If you make her human with the focused study alternate racial trait, get the Social Grace and Skill Familiarity Social Talents and possibly some customized skill-boosting magic items, she could be the party's male torchbearer one day and distressed granny the next with them none the wiser. (Honestly, the magic items would likely hinder her disguise more than help, as she'd have to explain to the party why she lights up when detect magic is inevitably cast when they all go dungeon crawling.)
Is she a tiny villain with invisibility? No. Is she particularly powerful? No. Yet she is capable of observing a competent party day after day while tending to her ultimate evil plan.
