Bats
What is your strategy for confronting the animal Bats in Dungeons & Dragons?
More info on this animal in an AD&D setting: Source 4
Swarm monsters are a bit trickier to fight. Many normal weapons are completely ineffective or do half damage. It is best to use a torch or to throw a lit oil source at them, spells are also effective.
One bonus to keep in mind is that you get an attack of opportunity on the swarm as it enters your space.
To win a fight with bats I think a lit torch is best. Though I wonder why a torch does damage but a sword doesn't.