I'll start by giving some hints about what you're aiming for.
×The mechanism at the end makes it essentially impossible to kill more than one waterskipper.
However, one will spawn almost right away, and once the force arrow is open, more than 10 turns will pass before the nest is killed, which means it will have a chance to spawn.
So, the key question is, how can you prevent a second waterskipper from forming?
Click here to view the secret text
×Make the first waterskipper block its spawn.
This means you need to have the waterskipper go down one path and then have the construct drop trapdoors to create a shorter path, so the skipper backtracks and blocks the spawn at the right time.
I'll leave you with that for now. Let me know if you still need something more specific.