Well, the bottom of the room clearly makes it pointless to try to assemble a full line of waterskippers in order. So you'll have to take advantage of the circular room shape instead.
The prerequisite is realizing that, because
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×the waterskipper is off-center, one square to the right,
the Soldier must therefore necessarily spawn
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×on the East entrance rather than the West. So, waterskippers must then be able to move Northeast into position.
This further suggests that, instead of making a full line, you want instead: (more direct spoiler)
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×one line of 4 waterskippers at the bottom, with relative movement order D-C-B-A from left to right. Specific numbers aren't so important. And, one line of 2 waterskippers just above that, with relative movement order B'-A'.
Building this setup is most easily done using the tunnels and a bit of finegling with the southern clone a few turns after the soldier spawns, to get those last two waterskippers into place.
The final trick:
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×Waterskipper A from the four-line must move before waterskipper B' from the two-line. This displaces the entire four-line North one square instead of northeast, but when the two-line slides against the northeast wall they rejoin a bit, then separate again, and finally everyone gets into position.
This pretty well spells it out, but let me know if you'd like a demo.
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Quick links to my stuff (in case you forgot where it was):
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