Anabstercorian said:
I'd throw a few very noticeable and powerful NPC groups that have distinct personalities. Not full parties - Just foils for the party, small, tight clusters of ne'er-do-wells that work together with confidence and trust. Some ideas.
The Gemini Triad - Three Devoted Defenders, each of whom takes another as his charge at the beginning of the battle.
The Alchemist and his Lackeys - A strange and powerful wizard who fights using several pistols, supported his many halfling lackeys throwing Acid, Alchemists Fire, Thunderstones, and Foottrapper readily.
The Invincible Warrior - A massive, silent figure in full plate that covers his entire body will frighten the players greatly when he takes unimaginable damage without slowing down. Of course, under that full body armor he's a troll fighter.
The Shadow and the Flame - A strange and disturbing Fire Elemental. The shadows that are cast by his light occasionally animate in to undead shades and attack his opponents, vanishing when they slay those who created them.
Eater of Minds - An Illithid Ghost who manifest directly behind his victims, immediately attempting to devour the essence of their mind.
If you do something like this, be sure to play up the defining aspects of the fight - Make it memorable, and don't be afraid to increase a creatures hit dice to make it more challenging. Don't nerf the players, give them something that'll scare the 


out of them!
Cool Ideas!
My suggestions, a party of 6 NPC's 7th or so level will do.
Make them 5 fighter/ rouges 4/3 with basic magic weapons and armor
A wizard with a hawk familiar plenty of scrolls and a good wand of Monster Summon 4.
and an evil cleric 7th or so domain should include travel and trickery
The trick is to ambush the party and use plenty of buff spells
Bull Strength, Invisibility, Cats Grace, True Strike, Haste
Have the wizard fly invisibly above the battle field summon a few Dire Apes or Arrowhawks. He is the sniper and should use magic missles flame arrow and that sort of thing. He needs to keep moving.
The fighters, carefully spaced and invisible or hidden each equiped with a potion of true strike and heavy crossbows with 4 or so magic arrows +1 sure striking that are envenomed. This will negate protection from missles.
In meele they will use spiked chains and the summoned critters will flank.
take down the mage and the cleric first. Keep moving.
The cleric. He will buff and heal as well as summon nastys to fight . He needs to stay static though. Basically he is a charging station.
Its not perfect but if the players aren't used to it. Well they will be in a world of hurt. Oh and one more thing.
Get the character sheets. All players are obligated to give the DM a copy of the character sheet. This is a must.
Hope this helps