I'm afraid the mad slasher and the beetle swarm will just about wipe them out, especially since they have no area effect damage ability. I don't have the adventure with me to check the base beetle size, but if it is Fine or Diminuitive, the party has no way to inflict damage on the swarm at all (it would be immune to all weapon damage and single target spells and effects).
I ran the mining office encounter posted here so the party had a scorll of burning hands(which they didn't use) and oil and torches. the oil and torches were used to kill the beetle swarm. It does say in the module I believe that lit torches will do 1d3 damage to the beetle swarm. I ruled that beetles soaked in oil took 1d3+3 dmg from a lit torch.
