In my current campaign, the group of would-be heroes wandered upon a Frost Worm. Actually, two of them - a mating pair. Though they weren’t mating at the time.
Anyway, the frosties resided beneath a Temple of Auril in a series of tunnels fashioned, over the years, by the worms’ constant burrowing. The underground complex was simply known as Auril’s Ordeal and had been carved by the giant worms themselves over the past few years. Ambitious priests, zealots, and condemned persons would enter the underground complex via a narrow, vertical tunnel. Those that passed the Ordeal – by surviving long enough to find their way out of the warrens – were considered especially blessed in the eyes of the Snow Queen.
Because I've already planned a Frost Worm attack, I thought it might be easier to pick that creature for a post on this thread. And if that wasn't reason enough, don't forget: Frost worms have been extremely ticked off, as a race, since first reading their descriptive text in the 3E Monster Manual and seeing the line, “Remorhazes are frequently the victors in such [frost worm vs. remorhaz] battles.” The frosties do have a point; a Frost Worm boasts a CR12 while a Remorhaz can only claim a CR7.
Frost Worm (CR 12) – Tactics Round-by-Round
Prior to combat: Frost Worms are excellent predators and extremely patient. They prefer to burrow underground at a likely crossroads, then wait patiently for unsuspecting prey (e.g., starry-eyed adventurers). Oft-times frost worms allow a thin coating of snow to cover their inert forms in order to better cover the entrance to their burrows. Frost worms particularly enjoy attacking passers-by during the night, thereby utilizing their Darkvision.
Round 1: If the bulk of the party is within 100 ft., emit a Trill (Will DC 17, failure indicates target is stunned for duration of Trill + 1d4 rounds). If the frost worm is successfully hidden, or if the party has not yet closed, it will ready an action to emit a Trill once a majority of opponents are within range. Ideally, the worm will wait until the party is near 30 ft. distant in order to subsequently use its Breath Weapon (see below).
Round 2: The worm will continual to Trill only if its enemies are still closing its position and have not yet entered the 100 ft. effective range. Otherwise, it will move forward in order to best position itself to use its Breath Weapon (15d6 Cold, Ref DC 22 half, those creatures held motionless by the Trill get no saving throw). In particular, it will concentrate on targeting those creatures that are obviously motionless from its previous Trill.
Round 3: The worm will now switch to simpler, but no less brutal, tactics. It will Bite any opponents within range. Worms are normally not smart enough to tactically select their target, so most melee attacks will be directed at whoever is nearest or has delivered the most damage to the worm in previous rounds.
Possible DM Fiat: Per the 3.5 rules, a Frost Worm has a space of 15’ (3x3 squares) and therefore could not attack while within its burrow (5’ diameter, 1 square), and could only move after a successful Escape Artist check. See Squeezing, pg. 148, in the PHB. Personally, I’d rule that the worm can function within its own tunnel as if the worm occupied a space of 10’ (2x2 squares), thereby allowing attacks (albeit at -4) and (restricted) movement.
Possible DM Fiat #2: I’m a bit confused as to the skill points indicated in the 3.5 MM. I think the worm should have 17 (1 per HD, minimum due to its low Int). The book lists three skills: Hide +3, Listen +5, Spot +5. Two points to Spot and Listen are from the worm’s Alertness feat. The Hide check gets a -8 penalty due to its Huge size (but also gets a +8 bonus when the worm is within its own environment). Whatever the math, it doesn’t seem to add up to me. Most likely I would treat the worm as a medium-sized creature if its body were initially hidden within its burrow, thereby removing the penalty due to size.
Cheers!
Destan
And to try to avoid making this just a "me too!!!" type of reply, let me put forward another facet of this that could be explored and written up: tactics for "weak" monsters to use against "tough" parties, i.e. some round-by-round tactics to make a group of kobolds truly a challenge for that APL 15 group you're running 
