Pack some cordials, get a decent gp food and you will get it eventually.

Start with Patience I and when you get a hook, use Powerful Hookset to get an hq Thalaos. Make sure that your gp will be sufficient from now on to spam Powerful Hookset for both the Thalaos and for a chance on Nepto, otherwise stand up, pop a cordial and repeat.
You will need to wait for the cordial cooldown to be able to use another set of Patience so you will need to rely on your gp regeneration at this point.

Hint: Try to memorize the thalaos bite (small-medium-large) so you can avoid hooking useless fish.

Good luck if you haven't got it until now!