It's a very little-known maneuver, and it's actually quite easy to do once you know how.
Basically, in a position like this:
.....W
....BW
.../.W
...\\.W
....SW
.....W
....WW
Move south. The slayer will retreat south. Move south again, and the slayer will retreat southwest. Then you can move south again and kill him.
Of course, it can be fairly unintuitive how to get into that position, but I can give you a demo if you'd like. Rest assured, the room is simply tedious afterward.
You can fix it by moving the room entrance one square to the left.
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