Mostly I'm just irked that players who make reasonable choices -- I'm a fighter in plate armor with a shield -- are just blown out of the water by players who know how to take advantage of every nook and cranny of the rules system.
I don't mean to slight you, but how many games of this have you actually played? A Zen Archer can be hell on wheels, but mixed in with 3 caster levels, I can't help but think that this character is all Barkskin and no bite. I wouldn't be too surprised if they monk takes little damage but also has a big damage output problem.
I'd much rather beat the offending player with a nerf bat than get into some arms race, contorting the game world and the narrative in order to justify enemies who can threaten her.
If that's what you gotta do. But I'd make sure it's a real problem here. The point Justin Brings up is very good. There are plenty of CR7 creatures that can threaten her before dipping into the special strategies some of the other posters have mentioned.
As for "contorting the game world narrative", I think if NPCs have a problem, they will deal with it logically. If the PCs present a problem, and they get a reputation, they will bring in talent that they know can deal with it, using strategies that they know can deal with it. It doesn't contort the game world narrative for an experienced caster to know that if the wizard/monk is banking +10 AC from spells cast at level 3, they are 1 dispel away from being a near sitting duck.
But I'd use this strategy carefully. I imagine that the player designed this character because they want to be untouchable. You say you have two fights a day. On the average, make one fight where she gets away with it, and one fight where the opposition has a few counters to her tricks.
Problem 2: Her character makes two other players feel weak and useless. Those players don't want to delve into the complexity of the bonus-stacking system of 3.5/PF, so their characters are weaker than hers. The system too easily allows you to make under-powered characters.
Does a lower AC make the other character underpowered if the monk's damage output is anemic?
Problem 3: Her character is not threatened by things that threaten the rest of the party. I, like the two aforementioned players, don't want to spend tons of time prepping for my game by designing custom foes. I want to grab all the handy pre-published foes and use them*. But most pre-published foes are built to target reasonable ACs. I feel it lessens the fun of the game if you're at no risk of defeat. Now sure, I could add foes who are higher level, but they'd just overpower the rest of the party.
*I feel that if you're going to design a game and release content for it, that content should be useful for the mainstream player base. Now perhaps I'm not mainstream for Pathfinder. I clearly want a system that doesn't demand as much time be spent on 'getting the numbers right.'
I don't think it's that complicated, but if you think it is, then you logically have to address it. Why exactly are you playing Pathfinder? If it's not working for you, is there another system that does similar things that will satisfy the players?
If they players want pathfinder for the specific options it provides, you may have to find a way to make your peace with it. Players here and on other forums can churn out some stats you don't want to deal with (but in most cases, I think it can be done by taking core options and adding one or two feat or spell selections.) You might try some of the rules tweaks for de-christmas-treeing pathfinder, like some of the options in Wulf's Trailblazer or in Green Ronin's Advanced GM Guide for 3.5.
If they players want rich character options and a d20-ish core but aren't stuck on pathfinder, you might try Fantasy Craft. It doesn't have wealth-by-level magic item assumptions.
If you want a simpler, more generic system, you might try Savage Worlds or one of the FATE variants.