Will M. Baker
As far as I can tell I have to remove all monsters in the hall!..
You're using Heart of Doom incorrectly. First, its Global effect is applied only when you enter the Dungeon, which you had already done when HoD entered the hall, so the effect wouldn't apply to you. Second, that effect destroys all monsters present (in your hand), not in the hall. So you're not forced to attack HoD.