[Lord of Nukes]

~Disruption Web renders a Dark Swarm completely useless. psionic Storm is also awesome against a Swarm.
~Plague and/or Zerglings are good to send into a Dark Swarm if you have them onhand, perhaps with some Ultralisks too to take fire before they get into the Dark Swarm.
~Firebats are ok, but it's unlikely you'll have a large expendable force of them. Irradiate the Defiler and some units inside the Swarm if you can. If they haven't got Melee units with them, just send your Marines into their Swarm.