Encounter: Twisting Tunnels of Terror

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I had an idea for an encounter that might be fun to design and run. I am not sure what campaign it would go in (I have a few campaign ideas mulling around in my head), but I am envisioning it as challenging a lower-mid level (3 or 4) heroic party of 5 PCs. I want it to be a challenging encounter - maybe 2 or 3 levels above the party, maybe a nasty part in a fairly traditional dungeon crawl and I would love an suggestions on how to make it work and not boring.

Setup: The players find themselves in a moderately sized room (4x4 squares). I want them to get there in a way that makes escape difficult, perhaps they are dropped from above, teleported in, or the door locks behind them. All around the room are a network of 1 square wide tunnels; these twist and turn and interconnect (forming a vaugley web-like pattern). Ultimately, the tunnels don't "go" anywhere they just circle around the room and allow it to be accessed from any direction.

In the center of the room is a large monster they need to defeat to escape. I don't know what it is yet or why they need to defeat it, i am willing to use a published monster or design my own. However, I want to have the following properties.
1.) The monster should be large sized - this prevents it from entering the snaking passages.
2.) The monsters should be elite and a few levels higher then the party.
3.) The monsters should be moderately dangerous in close combat but nastier if it gets to make a ranged attack.
4.) The monster needs to be really tough, possibly favoring defense over offense - the encounter won't work if a round of concentrated fire reduces it to a smoldering heap

So far the situation seems simple enough for the players: melee characters stay in the center room and pound on the big guy, ranged characters hit and run from the tunnels.

However, their is a complication. In the tunnels I want their to be a group of small highly mobile monsters (kobolds with their shifty ability would do well). These creatures dart through the tunnels looking for any opportunity to snipe the weaker members of the party and generally make a mess out of things.

This generally leads to a bad situation for the players. On the one hand, they need to kill Mr big and keep someone close enough to him to prevent him from frying them with ranged attacks. On the other, this would be a hell of a lot easier if the those little guys didn't keep popping out of no where to take pot-shots at the squishier party members. Do we take of the big guy or go and fight the highly mobile monsters in the constrictive tunnels?

My questions and concerns are
1.) I want the encounter to be challenging, but not so frustrating as to be unfun. I don't know who will play this but most of my gaming friends are mature players and can handle some difficulty.
2.) TPKs aren't fun for anyone and I can see this happening - one thought I did have was make this a capture, not kill, encounter. If the players are taken down they are taken prisoner (and can escape) instead of being killed.
3.) What would be a good monster for the center and what would be good for moving through the narrow tunnels?