3.5 confrontation / battle please help!

[Chester]

As a new-ish DM, I'm a bit sketchy on challenge rating. I could use some help setting up a decent encounter for Wednesday night.

Group consists of 4 level 3 players -- Bard / Rogue, Barbarian, Cleric, Dread Necromancer. All human.

Party has made some enemies with a powerful organization that regulates arcane magic. Particularly they want to execute the Necromancer and arrest the others (long story short, they know too much).

This will be a confrontation. Cleric will gladly surrender the Necro, but Barbarian won't allow it.

I need a mix of soldiers / magic users to give the party a challenge. (Maybe duskblades?) Party is rather strong--point buy with 41 points. They should fear death, but not be wiped out.

No need to overwhelm them with spells, but some arcane magic would be nice.

Can anyone help to suggest an NPC party to fight the group?

EDIT: Confrontation will occur along a road with some tree / boulder cover and some change in terrain height.

[atomicwaffle]

The institution that regulates magic sends their attack dog after them. In this case the attack dog is a displacer beast.

[Flickerdart]

How is the party's ranged weaponry? An orc barbarian 1 riding on a hippogriff and throwing down nets to scoop up the party would work pretty well. As soon as they get purchase, they take off into the air, so cutting free means they tumble down to their doom...unless they manage to climb up the rope and kill the rider.

Each rider+mount is CR3, so you can throw 3-4 against the party and not overwhelm them.

[Chester]

The institution that regulates magic sends their attack dog after them. In this case the attack dog is a displacer beast.

Actually a fine idea....

If I mix in a displacer beast with some human soldiers, what level / HD, how many, etc?