...#.
.../.
...#.
...#.
..@#.
When the monster appears in the door, if you shoot at it with shotgun from this position, it won't fly backwards because there's a wall behind it. And if it'll survive, you can move upper left as well as reloading the shotgun (Shottyman is required), making a monster not do its ranged attack, to make like this:...#.
.../.
..h#.
.@.#.
...#.
And then, blasting it with a shotgun can throw it back one tile, making it stop at the door tile, so if it'll die, it will leave no corpse.