According to the notes I picked up, magic will work on the Bulwark Beast without you needing to break its armour.
I've heard from a friend that if you've revealed the boss room, then talk to the smuggler, he'll give you some information on how to beat it. Not checked this for myself yet though.
Also, use gatestones often. I don't normally use them in small dungeons; but they're so cheap and save so much time that they're invaluable in larger ones.
Healer familiars not only heal you with their scroll ability, but make the food you eat heal more. Most people don't seem to realise this.