Well, I'm not a regular PF player, but have played 3.5 with high AC characters (both as player and DM) a number of times. A few things come to mind with making the mooks more effective:
1. Aid Another. Use it.
2. If they need a 20 to hit, go for broke. Power Attack. Rapid Shot. Who cares how big the penalties to hit are if you need a 20 anyways. Do things to throw more attack rolls or increase the damage that the few attacks that do hit do deal.
3. Friendly spellcasters. Something often overlooked is how much a fairly low-level spellcaster can buff low-level mooks, if given an opportunity. Look for spells that affect multiple friendlies. Magic weapon on a bucket of arrows that each mook takes a couple of?
4. One-shot magic items. Similar to the spellcasters, a few one-shot magic items (potions, scrolls, etc.) can make your mooks more effective.
5. Alchemical items. There are lots of things you can do here. They're fairly low-cost, and if you've got a mob of mooks, you can spare a few actions to do some different things.
6. Mobility. Use a big area and make sure that the mooks have room to maneuver, staying just outside of his melee range. They can also utilize superior movement to just go around the high-AC target and go for easier ones (I don't imagine the wizard is going to like being the only target the 10 archers can hit). Similarly, hitting the high-AC character with stuff that limits his ability to move around works too.
7. Take advantage of the difference in the number of actions you have access to with mooks. Keep some ready actions to deal with the pesky actions that the figher (or his spellcaster allies!) may try to use to wipe out a group of mooks.
8. Traps. Especially a water-filled pit. Very deadly. Let the mooks do something obnoxious like bull-rush. There's lots of material here.