I think that's not that they meant, but - and I agree on that part of the bestiary is odd - that you would need to kill 50 nymphs (which is highly unlikely) to get all the data in their entry. like attack, defense, drops, etc.
You only need to kill one, for the bestiary to register it, and that counts toward the zoologist's items, not whether you completed said monster's entry with getting all the data.
So, you don't need to hunt 50 of them them to get 100% completed bestiary, as far as entries (and so, rewards) concerned. but you do need it, if you want the data. but it makes no sense with rare enemies. because it's highly unlikely anyone would hunt 50 nymphs in a playthrough. these extremely rare enemies would better work the same as boss entries which get filled out completely, once said boss is defeated in the world once.