Party killers:
13th level human monks get six feats, plus three bonus feats.
Monk 1: Grappler. Monk bonus feats: improved grapple, combat reflexes, improved trip. Other feats: Jotunbrud, snap kick, superior unarmed strike, weapon focus: grapple, earth's embrace, improved toughness.
Monk 2: Stunner. Cobra Strike variant monk from UA bonus feats: Stunning Fist, Mobility, Spring Attack. Other feats: dodge, weapon finesse, ability focus: stunning fist, Improved stunning fist, weapon focus: unarmed strike, elusive target. If the PCs are light on casters, maybe throw in 1 level of rogue and one level of scout, so you get +2d6 damage whenever you spring attack the poor fellow being grappled by Monk 1. Heck, add in 3 levels of assassin for spells and another 2d6 damage.
For the teamwork thing, look into teamwork benefits from... PHB II? Heroes of Battle?
Assassin CR 8: ranger 2/rogue3/assassin 3. Feats: PBS, Precise Shot, Rapid Shot (bonus), TWF, Quick Draw. This guy throws lots and lots of daggers at grappled or stunned opponents. With a +6/+1 BAB, Rapid Shot, and TWF, his full attack has four throws. Each throw is potentially 1d4+1+4d6+Str bonus damage (plus poison). Have his Ranger favored enemy be the most common race among the PCs.
Cleric CR 7: take the trickery (access to Confusion) and evil (access to unholy blight) domains. For feats, take Spell focus: enchantment, Divine Metamagic, Heighten Spell, Scribe Scroll. Spend that NPC money on cheap scrolls (like buffs) and a few powerful/expensive scrolls (like Confusion and Unholy Blight). Cleric's job is to spam Blights and Confusions on the party. Blights can cause the Sickened condition, which lowers saving throws, which is bad news if you follow it up with a Confusion or three. Save one high level spell slot for a high-level summon (summon something Evil, so it benefits from your domain ability)--that way, you can start the fight with a summoned critter handy. Also keep a few dispels at the ready (can be scrolls).
Wizard CR 9: Conjurer, but make sure you don't bar Illusion and Transmutation. Remember you get Scribe Scroll for free; in essence all scrolls of spells in your book are half price. Feats: Spell focus: conjuration, augment summoning, greater spell focus: conjuration, cloudy conjuration, improved toughness. Wizard's job is to divide the party with walls, illusions, and solid fog so that the monks and assassins can do their work. Also, to summon monsters to distract/deal damage. Pick monsters that grapple or otherwise incapacitate. Use scrolls as needed. Also, if the party magics up the wizard should have a dispel magic or two handy.
Minimum buffs from Cleric:
1st level: Protection from Good on all (protects vs. mind control, helps with AC and saves)
2nd level: Str buff for grappler, Dex buff for stunner and assassin, wis buff for self, int buff for wizard
3rd level: Nondetection on assassin (protects against See Invisibility and other divination spells)
Minimum buffs from Wizard:
Mage Armor for monks, fly and haste for assassin, enlarge person for grappler, fly and expeditious retreat for self. Keep a few Improved Invis spells on scrolls in case some get dispelled. Also keep a Dimension Door at the ready.
This party is toast.
-z