Spoiler Alert! We have some more information about how to perform this quest but recognize you might want to try it on your own before we tell you the secret...
The eggs float around the edge of the water, and the nests are found on the bottom of the pool near the waterfall at the base of Sardo Castle. Just jump into the water at the Kyosa Village bridge and swim east.
The eggs are easy to destroy (about 10k HP) but the nests take much longer (around 155k HP). On the bright side, these are mobjects, which means they are not mobile and do not fight back, and there are no aggressive mobs in the vicinity (unless you blunder into the frogs on the shore).
