The best way to get this trophy is to get the Retrial skill from your Spellworks. After that go to the Kingmaker shrine over the mountains near Goldpaw. Then fight the boss there. The fight should be familiar to you. Just wait for him to summon the rocks to fall on the platform your standing on and then break them to yield 2-3 balls each. You can extend the battle for as long as you'd like for him to keep summoning them, it's important you do not activate the Hiddledies that come out of the rocks as I found it delays the rock-summoning phase.
I did this after completing most of the other trophies and it only took me around an hour and a half to get it.