This room is filled with a vast number of dwarves, which are on an insanely high respawn cycle, with many patrols.
On the far side, the doors to The Iron Hall stand locked and unpickable. The only way to open the doors is to kill the two flamekeepers on each side of the room, then use the torch to light the braziers on each side of the door.
The torches from the flamekeepers only last 5 minutes.
Send one or two stealth classes into the room, to kill the flamekeepers while the rest of the group works through the room to the braziers.
VERY hard without serious AoE-power.