Quick elaboration.
Place the trigger enemy in a room with a shutter door that you want activated from elsewhere in a dungeon. Then, populate the room as usual with that trigger enemy in place; however, add the "boss room" screen flag to the screen.
Later, set up a dungeon boss room as usual, complete with boss flag.
What happens with this: the boss flag, as we know, activates on every screen on which it is active after all enemies have been defeated (hence why having two bosses is impossible). Hence, after defeating the boss, you return to the room where the trigger enemy is placed; since this screen has a boss flag, the trigger enemy no longer exists and the shutter opens.
Now, the last paragraph brought up an idea; how about an "enemies never return" screen flag? This can allow setup of, say, a sub-boss room.
Cyclone
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