The trick is to get all the goblins to be west of you in a bottleneck, with no stray troublemakers to the east. That bottleneck is created by yellow doors at squares
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×(14,22) and (14,23).
More specific instructions with room image:
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×
You'll be running around the room counterclockwise twice. The first trip is to drop three of the four trapdoors [at (1), (2), and (3)] and hit the orb (4).
The second counterclockwise trip is for killing goblins. If you go straight to the main bottleneck, you'll get ambushed from behind by 4 goblins.
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×So cross through the arrows at 5 [that's square (2,12)] and hide in the southwestern box with the checkpoint (6) [that's square (6,22)]. Kill the goblins that follow you in there. While you're doing that, the 4 troublesome goblins will have moved west and joined the rest of the crowd. Come out and butcher goblins at the bottleneck (7).
Hope this helps
