More wizard hunting (witch-hunt?) tactics for Mr, Jacobs. These are lesser means I'm sure you've considered but are worth mentioning. For starters, I'm hoping Iggwyvl isn't an outsider or has any type other than humanoid. I have found that being undead or an outsider at high level tends to be a liability. I think about Lashonna, and the Maximized Bolts of Glory fired at her that wiped her in two rounds, Mass Heals thrown at Kyuss which simultaneously hurt him and negated any damage he had already done, or the numerous Anti-dragon spells employed against Brazzemal the Burning.
There is always the freak chance that a couple of magical rays, like disintegrates, bolts of glory, and the like get through her resistance and score a critical hit doing massive amounts of damage. She's probably got the HP to survive a maximized Disintegrate or two, but I'd hate to hear about her going down because of a couple freakish rolls. Give her a Luck reroll or a Ring of Wishes to negate the one in twenty chance she critically fumbles a roll. This works good for DMs who like to roll openly (like me).
Also, consider the old "I ready to disrupt as soon as she starts casting" tactic. I've seen this completely nerf wizards who never got a spell off because every one they try and cast is countered by an arrow, fireball, or some other distraction that takes a DC 40+ concentration check to make. This is the case of multiple actions of powerful PCs versus the actions of one bad guy, and without either dual actions or a substantial concentration score, she could be in trouble. I'm sure her concentration score will be in the 40's anyways, but its something to consider.
Finally, consider the terrain where the PCs encounter her. Give her every possible advantage to the point that no PC could just charge her in round one (The Harbringer comes to mind). I have found that depending on how the BBEG is setup, players may be less likely to attack if they feel they can't do something immediately to her without her getting a spell off. Hope this helps a little more, and good luck on crafting the Queen of all Witches!