Okay, how to handle...

[RUMBLETiGER]

"A weapon with a special ability such as flaming or frost deals its full energy damage with each it, even if the weapon's normal damage can't affect the swarm."
Enchanted weapons with energy types.

 

[frankthedm]

Hitting it hard with a blunt weapon sounds like the most effective method. Web will slow it down for a couple of rounds, I think.

Nasty critter. Watch out for the parent.

 

[Grogg of the North]

Web followed by Acid Fog would be my thoughts. Otherwise, if the swarm is unintelligent send a couple big fire elementals at it. The swarm will burn itself to death on the fire. If it doesn't, call shenanigans on the GM and hit him/her with your dice.

 

[krupintupple]

It's a barely intelligent swarmy monster, so it won't be playing tactically at all. My suggestion is to slather a pack mule, or horse (could be summoned) in barbeque sauce and run like heck the other way. If your DM persists, remind him that frankly, a CR12 swarm with an SR22, Ref +22, and evasion is fairly stinky cheese for a group of CR10 heroes. It's not impossible, but unless you've got the ability to deal with a very specific set of hindrances, it could easily become a TPK.

Failing that, Swarmguard Armor from Dungeonscape may take some of the bite out, as will being warforged. Otherwise, ask him what he would've done, stroke his ego to see if he actually had a genius combination of tactics that were feasible, given the resources of the group. Or, just to see if he's a sociopathic dink.

 

[Dannyalcatraz]

Find a bottleneck and use a barrier spell (Wall of Fire, Wall of Ice, etc.) of some kind. Or, barring that, use barrier spells to create bottleneck. If nothing else, it will buy you time.

 

[RUMBLETiGER]

How about a lot of little glass jars, filled with meat sauce? have the lids handy. Roll your players around in flowers or mud or skunks or something, so as to not smell like the meatbags they are.
As each little creature enters the jar, close the lids really quickly. Or drop a really big rock on top of them. Or a large wooden board to seal the jars, in case you can't smash them.

Or how about have the Paladin say, "You go ahead, I'll hold them off, save yourselves!" while the rest of the party runs away. Then, either laugh at the Paladin for, well, being a Paladin, or pray to his deity asking that he be rewarded for his heroic sacrafice, whichever is more your party's alignment inclination.

 

[HiddenLineage]

It's obvious what you should do. Here it is in a few easy steps! Then you too can be on your way to a swarm-free tomorrow!

Step one: Stand up
Step two: Grab MMIII
Step three: Beat the DM senseless with it
Step four: ???
Step five: Profit

 

[Dannyalcatraz]

Wall of iron, toppled, should crush a lot of 'em.

 

[Hassassin]

Swarms with Int aren't immune to mind affecting magic. The one in question only has +12 Will so it has a reasonable chance of failing a save, too.

 

[krupintupple]

Swarms with Int aren't immune to mind affecting magic. The one in question only has +12 Will so it has a reasonable chance of failing a save, too.

Wait, so ray of stupidity/idiocy (2nd level arcane) will drop the entire swarm in one shot, sans save? What?