Giant Yellow Nails are from Bronzed Brendal not regular Grendal. You can also do Sand, Heat, and Giants.
I would suggest practicing against regular Grendal as Bronzed Grendal only has 2 moves different from regular Grendal. I used to get destroyed by Bronzed Grendal until I ran Crushing Blow a billion times and I got used to the movements. --- PSN: RyuKenri