I followed your advice Olaf - made the bat swarms immune to weapon damage.
Fortunately the party worked in concert to take out the bats with a few well-placed vials of Alchemist's Fire.
A few Goblins to beat their way through, a couple patches of caltrops, and one of the Goblin Adepts. This encounter was fun, and the party stayed focussed - at least until all of the Goblins were dead and they thought about hanging around and collecting ears at 5 gp a pop. They escaped into the sunlit bath house just ahead of Drathkar's yellowed fingernails!
Lots of highlights, and my favourite: Gus, the half-orc Barbarian/Cleric succumbed to a sleep spell on his way to the Adept, just before being 'coup de graced' by a Goblin Skirmisher Gus awoke - thanks to the point of bleed damage he took from being chomped by bats!
After extensive preparation, the party re-enters Drathkar's way at the end of the month. They've enlisted the aid of Rufus and the surviving 'Pulverizer Automaton' from Jazidrune (which is a little more self-aware than it used to be - must be all the time it spent surrounded by those arcane forces! - I'm hoping someone takes it as a cohort)