Steal all the barrels from the zhents and blow up the entire pit at once. Make sure your party is stealthed up in the rafters. Goblins can't be upset at you when they're dead (and also didn't see where that alchemist's fire came from).
But realistically, everyone else is right, get her alone in her room, and it should be easy. If she gets a turn she'll turn other's hostile, so either stun, silence, or just kill her in one round.
