Regardless of how many tricks you give your villain, keep in mind that she will still be limited to one action a round without extra help.
This means that your villain must:
a) have the requisite "big six" boosting items
b) have cast all of her buffing spells prior to their arrival
c) must have freedom of movement, both the spell and the ability to move about the battlefield
d) once in melee, she must be able to hit hard, or be really good at some form disarm/sunder/trip/grapple
This is about the only way that ONE character will be able to take on a party once they surround her.
I thought you might be able to use Enlarge Animal to mess up the PC's, but as per RAW, a familiar is considered a Magical Beast instead of an Animal.
Bottom line... you're the DM, and you can make up anything you want to challenge the PC's.
I would suggest Rogue 3/Monk2/Chameleon3.
* Max out Wis for the Monk's Stunning Fist; if the PC is stunned, the villain will get her Sneak Attack damage on every subsequent hit.
* Because Monk and Rogue get Evasion, the villain would get Improved Evasion for free.
* Monks are built to do special attacks; disarming PC's will drive them nuts, and tripping is just awful because being prone denies AC, which means more sneak attack damage.
* Monks can hold items in both hands and still fight at full capacity due to their fighting style. A couple of Wands would be handy, especially one of Dimension Door.
Optionally...
A Psychic Warrior2/Monk3/Chameleon3 would be interesting as well with the right psionic powers... expansion, inertial armor, and maybe burst, biofeedback, or metaphysical claw. Use the PW bonus feats to add powers if you want.
Ultimately...
She's a handful of hit points and a pile of treasure to the players, so don't get too attached.
