This achievement can be completed by creating any custom empire with the "Anglers" Civic. This civic adds the aquatic trait (and a preference for ocean worlds), and cannot be used with certain origins.
As an angler, wait until you have enough fleet size & provoke a Fallen Empire:
- Diplomatic Insults
- Settlement of worlds in FE territory
- Using a Colossus on a FE holy world
(there are a few other ways, but usually settling near a Militant Isolationist or colonizing a Spiritualist holy world will guarantee a fight...unless your fleets are much more powerful that the FE)
The war goal can be "Humiliate" (this may be the only option... it was for me).
I beat their main fleet one time, won the war, and had the achievement pop up right after.