I usually start by having someone sneak to the top floor, then throw a fire spell at the fireworks to kill most of the npcs up there. Aside from enemy fireballs going through floors, I also hate that sometimes the Steel Watch outside will sometimes try to arrest you in the middle of the fight.
.... which they also do because the building's floors/walls apparently don't block LoS and thus NPCs have x-ray vision on you. Or at least some supposedly solid surfaces have that LoS-problem.
My usual approach here is also much more blunt and brutal: Put someone outside to shoot through the open window in the top floor, put a smokepowder arrow through it and watch the fireworks. But this time, I had honestly forgotten about the glass-house nature of the building and I also wanted, for once, to do things without setting off a huge explosion on the top floor.
Oh well.. now I cheeze-loaded the game and went straight to the Ansur-quest - Toys, Ansur and the Gortash-fight are the only things before the end-game left to do on my play-through and since the toy-quest isn't really leading to anything meaningful, I'll ignore it this time around. Don't need the money/loot and certainly not the XP at this point, so ...

S.