Grey Elf Wizard 5/Incantatrix X
Class Features and ACFs: Elven Generalist (RotW) and
Domain Wizard for lots of hassle-free slots and freeing up wealth. Moth familiar (dragon) for +6 spellcraft. I also like Spontaneous Divination (CC) with this setup, even post-errata, but that's more for out of combat versatility.
Feats: Extend Spell, Persistent Spell (CA), Ocular Spell (LoM), Chain Spell (CA), spellcraft boosters (such as Skill Focus: Spellcraft and Shape Soulmeld: Mage's Spectacles [MoI]) if necessary
Items: Headband of INT, Spellcraft boosters, Ring of Enduring Arcana (CM), Orb of Bodily Renewal (MiC), costly spell components, incl. rubies (see below)
Spells: Craft Magic Tattoo (SpC), Fly, Haste, Greater Invisibility, Suffer the Flesh (MoE), Dolorous Blow (SpC), Polymorph, Heroics (SpC), Battlemagic Perception (HoB), True Seeing, Greater Magic Weapon, Superior Resistance (SpC), Greater Mighty Wallop (RotD), other ones I'm not thinking about right now.
Step 1: Jack up her Spellcraft Check. Moth Familiar, synergy bonus, circumstance tool, and if necessary an item or Mage's Spectacles should obviate the need for an item familiar.
Step 2: Jack up her CL, especially versus Dispel. Persist Suffer the Flesh for +5 CL. Orb of Bodily Renewal removes the CON damage. Extend Create Magic Tattoo for +1 CL. Always wear the Ring of Enduring Arcana on for another effective +4 versus dispels.
...Step 2a: Throw up Battlemagic Perception as well. Prepare enough Dispel Magic and Greater Dispel Magic to be able to counter their Dispels when they inevitably come.
Step 3: Jack up your teammates' power. Put some combination of the rest of the above spells on
everyone. If everyone is invisible, and flying, wielding +4 Greatclubs that deal 8d6 damage, and polymorphed into War Trolls for the irony, the Incantatrix should now be the target... if they can actually target him. (How do you Persist Polymorph on everyone, you say? First you make it Ocular, which gives it fixed range, which makes it Persistable. After all that you Chain so it affects everyone.)
...Step 3a. Use extra Metamagic Effect uses on personal defense, since the Incantatrix and/or her spells are now the target. Greater Mirror Image and Greater Blinking are two wonderful spells with some decent utility as well.
...Step 3b. The other casters likely have good spells for you to use Cooperative Metamagic on. Use them, obviously.
Step 4. Decide what she's going to do with her actual actions in battle.
...Step 4a. She's buffed up enough to go into melee if you please; just throw on Wraithstrike, Bite of the Were[animal] (SpC), and Malevolent Tentacles (there are more, but you should probably use your other ME uses on your allies) and she's a killing machine.
...Step 4b. If you want her to stay back and blast, your best bet IMO is to nab the Outsider type somehow (otherworldly will do it) and use that Polymorph to turn her into a Kelvezu (MM2). Large DEX, large Natural Armor Bonus, massive sneak attack. After that you just persist however many Cloud of Knives you can and use your standard action on an Orb of Acid or whatever. BTW, Acid Sheath + Orb of Acid is a fun combination.
...Step 4c. If you're really dedicated to keeping those buffs up you can dedicate the rest of her actions as readying to counterspell. This also means she doesn't outshine anyone in combat directly, and enemy mages are quite neutered with the right feat choices. She
does have a pretty marvelous caster level for those opposed CL checks. Note that there are other ways to get rid of the buffs, including Antimagic Field (lame), Pierce Magical Protection, and Dispelling Weapons.