It's not that much different to a regular solution, just you need to corner the goblins against pits instead of walls. There are at least a couple of approaches that work:
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×You can close off the top 3 squares on the western edge and then charge in the middle row for a bit, before backing off to the western platform using the 4th row. If you do it correctly, most of the goblins will be stuck in the top alcove you just created, a couple might die when you charge back in on row 5, and there will be one you can sweep around into the alcove . Clearing a 2-wide alcove full of goblins should be reasonably straightforward.
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×You can build yourself a little alcove instead, by stepping on the NW-most trapdoor and then S, SE, N, E, NW. That will let you kill a couple in place and sweep the rest around onto the western landing. From there, you can run east along row 4 (creating a 1-wide bridge in row 5), run back along row 5 to kill most of them, and you have one final goblin to manipulate using the last few trapdoors in the SW corner.
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