The Fonadu is the first Fiend Intel you will get when you reach the Junon reigon in Final Fantasy VII Rebirth, and this bird is indeed not easy to pressure if you are looking to meet all the requirements in this Intel Mission. But the simple answer is, you have to dodge its deadliest attack.
Monsters in Final Fantasy VII Rebirth get pressured either by exploiting their weaknesses or by a perfect dodge and/or a perfect guard. Fonadu is weak to wind attacks, but this is not how you can pressure it. You actually need to dodge its Death From Above attack, which you can trigger by exploiting its weakness. A few casts of any of the wind-based attacks will trigger this attack against you, so you need to be ready right after because it is a fast sweep. The mission also asks you to stagger them, so don’t forget to continue attacking them afterward to finish all the mission’s side perks.
Watch the video below to see how I did