If your opponent is dust, then obviously the solution is fire!
You whip out your lantern and brandish it like a shield between you and the beast. It cowers back, just like you predicted! Though it growls in protest, you drive it back towards the closet with short jabbing motions. Each time you do, the metal body of the lantern squeaks and swings on the hinges of its handle.
Just as you're about to declare victory over the monster—
*crea—SNAP!*
—the handle breaks off your lantern, sending it sailing forward into the dusty body of the beast.
You propel yourself back as far as possible as the monster bursts into flames. Luckily there's not enough dust left in the rest of the room for it to spread, or else you'd be really worried. Still, the victory is yours!
Admittedly it's a lot more violent of one than your original plan of forcing the monster into a closet and barricading it in, but if it works it works!
That train of thought holds up for about five more seconds, which is just enough time for the dust monster to stand back up. Its body is little more than roaring flames now, with a screaming skull still glowing away at the center.
With a demonic bellow, it charges you.
"Oh bu—!"
[A fiery end to a fiery soul. Add one (+1) to your Fear Meter]
"I think that may have been your worst plan yet."
"I know."
"I mean, honestly, who doesn't know that you should never set a lapin particulus on fire?"
"I didn't."
"They are a magical cousin of the Timberwolves. Even as their material components burn, their magical essence will allow them to keep moving."
"Yeah. I noticed."
"I didn't even prepare anything special. I never even imagined that somepony would try that!"
"Well I did and it's done and can we move on already?!"
"Oh certainly. Feel free to leave whenever you like. I'm just amusing myself. There's nothing keeping you from continuing on. Though do try not to get too much soot over all my decorations."
There doesn't seem to be anything to interact with in the foyer, but there are three hallways that branch off that head LEFT, FORWARD, and RIGHT.
If your Fear Meter is full, you must select the fourth option.