Others have given a lot of good suggestions. Here's a few more.
Wizards have limited spells/day, even at 11th level. If he's triggerhappy, let him see a Major Image of a band of gnolls or whatever troops the temples are tossing at the group regularly coming around a corner just distant enough that the fighters can't get there in one round. Let him toast them with a fireball. One spell down, no bad guys hurt. . . .
Remember Spell Immunity. Give some of those clerics Spell Immunity: Fireball, Magic Missile, Cone of Cold and then have them prepare with Protection from elements Electricity. This will work especially well if he's got a reputation for using a couple of elements. Alternately, give a few clerics brooch's of shielding for magic missile and let them use spell immunity for all his attack spells.
High level clerics could even Commune before attacking the party to find out what elemental types he's got prepared that day. If he's only got one or two prepared, the clerics could cast Mass Resist Elements (Tome and Blood clr 3) on their hordes.
I imagine the temples have a wizard somewhere or other. Have him start out invisible and hasted and ambush the party by throwing up a wall of fire (circular around the party) and then follow up with a wall of stone (around the wall of fire). Well, maybe that wouldn't be so fun for the players. . . .
It's the temple of elemental EVIL right? So have the bad guys send their grunts into the slaughterhouse in small groups for him to fry. Meanwhile, an invisible assassin is studying the party's main artillery (the wizard). Three rounds later. . . "That's 32 points of damage from the assassin's sneak attack. Make a fort save or Die! Oh, by the way, the blade had purple worm venom on it. Make a fort save (DC 24) or lose 1d6 strength. . . . Now the assassin makes his second attack. . . . using his extra partial action [from his boots of speed] the assassin pulls out a wand [of invisibility] and vanishes."
Barbazu can teleport without error at will. They are also within the hit dice range for lesser planar ally (actually, they could have fighter levels as well and still fit under the hit die cap). A hasted Barbazu could teleport in next to the wizard and attack. On its next turn, it attacks and uses its extra partial to teleport out. Give the Barbazu above average strength, dexterity, and constitution and the combat reflexes feat and see what happens. . . .
Spell Resistance is a 5th level cleric spell.
Since the party is apparently 10th or 11th level, this would theoretically be a reasonable encounter:
Troll Monk 6: Str 24, Dex 16, Con 22. . .
Feats: Combat Reflexes, Large and In Charge, Lightning Reflexes, Iron Will.
Of course, he's only CR 11. To challenge an 11th level party, you should add an 11th level cleric to the mix.
Before the pair attacks the party, the cleric casts Energy Immunity: Fire and Protection from Elements: Acid on the troll. Protection from elements: Cold and Fire on the cleric. Bull's Strength, Endurance, Owl's Wisdom, and shield of faith, and magic vestments on both of them. Both villains down potions of dexterity. Death Ward on the troll. He casts Spell Resistance on himself.
And the cleric starts the attack off by using a Summon Monster VI to summon a fiendish dire bear to attack the party. (Assuming that you allow template swapping for summons).
