You already know the room. The creature is the constant. You are the variable.
The creature. Not the room. Bosses have their own page.
Slayer · RuneScape
Where
Ancient Cavern
Assigned by RuneScape slayer masters. The live page lists who.
Hunt
Magic. Protect Magic. Crush helps. Ancient Cavern is the room.
Watch
Task-only doors, face covers, and cannon-blocks are on the live page. Confirm on the wiki.
Kill
A kill ends when waterfiend drops. Walk the room if you pulled extras. Reset at the door when the task is done. Wrong prayer or the wrong tile is the wipe. That is the whole file. Do not treat this as an enrage boss. Those sheets live under Bosses. Counts, drops, and the living method: the RuneScape wiki for this hour.