Okay, how to handle...

[Greenfield]

In our D&D 3.5 game we recently faced a Swarm creature from MM III. I think it was called a Brood Keeper Larva Swarm.

At something like 170 hit points, made of Tiny critters (I think), DR 1/-, 1/2 damage from slashing and piercing attacks. Spells that target a single creature, a specific/limited number of creatures, will have no effect on it. SR 22

It has a +22 Reflex save and Evasion, and something like a +17 Fortitude Save.

Fast Heal 1. Spot and Listen +15 each, 60 ft. Darkvision, if I recall correctly.

It does 5 D6 of damage per round to anyone in the area, and has the Distraction capacity, meaning any spells to be cast from inside have a base Concentration check of 20 + spell level, before you count the damage.

Flight 30, with the "Run" feat, so lots of movement.

You are a 10th/11th level party.

How do you handle such a thing?

 

[Crothian]

Move faster then the slower members of the group.

 

[the Jester]

3e swarms are one of the only times the old reliable fireball type spells really come into their own, if only because not much else works.

Another option along the same lines as Crothian's: Summon something to feed it.

 

[RUMBLETiGER]

my first thought was the Fiery Burst reserve feat. Endless, repeated blasts of area fire damage. Not technically a spell, so can it be used despite the Distract ability?

Gust of Wind is a classic fallback for dealing with swarms in general.

Be a Warforged spellcaster, and be immune to Nausea.

Check out this thread: Winning Tactics Against Swarms.

The SR is kinda evil. Sounds like you need a Factotum level 11 with Fiery Reserve.

 

[Meatboy]

I guess all fighters and rogue types need to run like heck while the casters fight this with AOE spells...

But if I had to fight it I hope that we are in a confined area like a hallway to restrict its movement. Use spells like waves of fatigue to make sure it couldn't "run" effectively or Wall of Fog to slow it down. or set up a wall of fire spell that it had to move across length wise to maximize damage.

The Cleric Spell repel vermin might work to especially if successive barriers are stacked. It requires a will save to pass the barrier and even if they do they take 2d6 damage. It could keep them away from the party while any wizard blasts at it.

 

[Greenfield]

The problem with both Fireball and Fiery Burst is that they both allow a Reflex Save, and the critter has a +22 Reflex Save plus Evasion.

Gust of Wind can carry the swarm a short distance away. Because their "attack" does damage by their simple presence around you, they can double, triple or even 5x move onto you, ignoring terrain modifiers because they're flying, and resume their attack that same round without missing a beat.

 

[Ahnehnois]

It's a tiny swarm so it takes damage from slashing and piercing weapons. The damage is halved, subjected to DR 1/-, not subject to crits, and is healed at the rate of 1/round, but the swarm itself only does 5d6 damage (entirely subject to any DR you might be able to come up with and far less than an 11th level cleric can heal in a round) and is not going to use any real tactics because it has 3 Int.

The fortitude save for distraction is makeable for a good fort character of this level, and your fighters are getting 2-3 attacks per round before haste, TWF, or anything else. Remember that elemental damage dice on weapons are highly effective against swarms and not subject to SR.

I'd say a couple good fighters with casters healing/enhancing them could take it. I suspect that a fighter of this level, even after all those drawbacks, could deal more than the average of 5d6 damage in a round and that a whole party with a healer would have an effective reserve of much greater than 170 hp. Make sure to stay far enough apart that it can't hit multiple characters at once!

You are a 10th/11th level party.

It is a CR 12, and a tough one, so I wouldn't expect an easy victory. This is definitely a really tough fight.

If your party is mage-heavy or just not up to the task, you're plenty high enough in level to be able to teleport, fly, or othewise escape from it.

 

[kitcik]

At 10/11th level, you must be able to do 170 HP damage in one round, even with the reductions.

My monk (MH1, monk 8, warblade 2) averages like 90-100 damage per round with a nice bard buffing him, and that's one suboptimal character.

This thing goes down in 2 rounds max with no blaster spells, so you only have to survive 10d6 damage (average 35, unlikely more than mid 40's). Arcane casters need to steer clear (which should be easy), and everyone else should be fine.

 

[Crothian]

The thing only takes half damage from slashing and piercing attacks so that just means the fighter or barbarian or whoever is the main melee guy pulls out his back up bludgeoning weapon and goes to town on this thing. It only as an AC of like 21 so even with a good amount of power attack one should be able to hit it easily.

 

[was]

Try Web, Stinking Cloud, Ice Storm (bludgeoning) and Acid Fog if you have access to it. I believe these bypass SR