My suggestion would be similar to ones already used, only a little bit different:
Attack him with a rog19/ass1 wearing a ring of blinking. Your opponent is denied dex bonuses vs. all attacks. Cast haste on yourself with a potion. Using the quickdraw feat, attack him with alchemists fires, at +14/+12/+12/+7/+2 BAB. Obviously, a 20th level PC can pump those up. And you ignore all dex bonuses, all armor bonuses, and all natural armor bonuses. Each successful hit does 1d6+11d6 sneak damage +1 (point blank shot) damage.
Note that the ring of blinking denies the opponent any dex bonuses and therefore any dodge bonuses (PHB, p81) as well as his uncanny defense bonus (S&F, p18). Mr. AC doesn't lose his monk wisdom bonus; anyone know a way to immobilize him with a touch attack? I'm thinking maybe you take the leadership feat and have several followers equipped with rings of blinking (and fire immunity) who then grapple Mr. AC; if a grappled opponent is immobilized, this will get rid of the pesky monk bonus to AC.
By my count, so far we're ignore 115 points of AC, counting only armor, natural armor, dodge, haste, and dex bonuses. I'm thinking he gets to keep his cha bonus from mystic wanderer, sadly.
Ooh! Ooh! A better tactic.
This time you're a cleric using shuriken coated with dragon bile. This is a contact poison, DC 26, that does 3d6 points of str damage. You can fire off 15 of these/round; within the first three rounds, you're likely to have two of them succeed at least on touch attacks. Drop the poor fellow to 0 strength, and then CDG.
If you want this strategy to be even better, take the leadership feat. At 20th level, you can easily have 163 followers, all fighters, each of them with exotic weapon proficiency (shuriken), point blank shot, and rapid shot. That means that each round, poor Mr. AC will be facing 978 shuriken. One in 20 of those will hit poor Mr. AC, meaning that he'll take 48 points of damage/round, most likely. And if half of them are poisoned, that means he'll need to make 24 saves, DC26, against a 3d6 STR poison.
You, of course, are delaying your action. Once the last of your minions has gone, you take an MEA action to move next to poor O STR Mr. AC, and then you CDG. Power attacking. Using a miner's pick. Coated in black lotus extract.
If it matters, you've cast tenser's transformation, righteous might, divine favor, greater magic weapon, and whatever other spells are appropriate to increase your damage. And you have the strength domain. And, of course, you're hasted.
I'm guessing that poor Mr. AC wouldn't survive long in this arena battle.
Daniel
(I'm also thinking that you weren't intending followers to participate in this battle, but hey -- that wasn't against the rules!

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