That never ending quest for more AC

[Mystery Man]

splash damage from grenade like weapons.

how are the PCs causing damage? if ranged... you use back. but bigger weapons or magic from cover/concealment. or switch to melee with reach.

Typically the ranger who can fire off 5 arrows a turn will hang back while the pally wades into melee combat. the sorcerer/spellsword and the arcane trickster will fire spells then go either ranged or melee. A force to be reckoned with really.

This is of course if you don't count last week where the paly was busy trying to wrestle his weapon back from cave trolls (long story) most of the combat while the rest of the party had to deal with a deepspawn (advanced). They were a bit off their game last time.

 

[Ourph]

Quicksand.

Quicksand = entangled and drowning at the same time.

Quicksand is bad, especially for people in armor.

Quicksand....works every time.

 

[Prism]

How about something large or even bigger and swap out one of the MM standard feats for improved grapple. After a round of combat, if they don't hit they could go for a touch and grapple routine which pretty much ignores armour. Something with improved grab already (so it does more damage on the grapple) and possibly constrict too

A swarm could be a nice monster to throw at them - no roll to hit

 

[painandgreed]

Nautical adventures. Nothing like the idea of sinking like a rock for half a mile straight down to make you wonder about wearing that heavy armor. Then add spell casters. Say these pirates have a wizard and a mind flayer for leaders. By time the ships get close enough for melee combat, the outcome will probably already be decided.

 

[Testament]

A nice big body of water. Sink or...wait, how much armour? Sink.

One encounter with Incorporeal Undead scares the heck out of AC monkeys. Not many people Ghost Touch their armour.

Grapplers. Works every time.

Spell-chuckers. Touch AC and saves.

Abberations. Specifically, Beholders and Mind Flayers. A Mind Flayer Telepath/Wizard could be really nasty.

 

[Al'Kelhar]

Abberations. Specifically, Beholders and Mind Flayers. A Mind Flayer Telepath/Wizard could be really nasty.

Falls. Into armour-eating acid. In the lair of a half-illithid beholder. With the half-dragon rust monster pet.

 

[Eldragon]

Incorporeal Undead versions of past party members, enemies, maybe even the monsters they fought last session.

Easy to include such creatures into the plot.

 

[Herremann the Wise]

Swarms can be down right nasty for high AC people, and Magic works well too.

AC is only one form of protection in the game.
Saves are another,
Spell Resistance one more,
Damage Resistance as well and then several other specific effects.

There are ways and means around all of these. Be thankful your PC's are only testing out your ability to get around their AC. I'm currently having similar difficulties challenging my 17th level uber group. It can be done and without too much effort but you just have to know the rules and obviously the ways around them. If you can give a few specifics, I can make a couple of specific suggestions. For the moment though, different swarms are nice.

Best Regards
Herremann the Wise

 

[Christopher Lambert]

Get some big grappler on them. A dire werebear (white) frost giant should do the trick. Ignore the alignment, of course. I came up with a grapple bonus of over +50, and this was a CR 15 creature I even ended up with a Will save of +15 (but rolled a "1" ... never brag when behind the screen).

 

[Castellan]

You know, another idea might be to change the kind of adventure you run... Take a look at Piratecat's Story Hour for some examples of how to use NPCs who can't be defeated through combat. Pit your players against a mid-level bureaucrat in the next city they visit. Not only is the guy only a 5th-level Expert (or so), but he's also part of the local government. The PCs could whallop him easily, but it's immoral AND highly illegal. And, the more intimidation the PCs try, the more red-tape he puts them through...

Imagine your heroic PCs having a warrant for their arrest because of "failure to pay back-taxes." No Armor Class in the world is going to keep them safe from the tax-collector.