If you've finished the game, you should have 3 battle zones. The first step is to basically beat up one whole zone. I find that sometimes trainers respawn after a bit, but get as many as you can out of the game. I recommend Gyarados for the ZA Royale due to its high power attacks and good coverage using Earthquake. Next, teleport to another zone and battle there. By the time you're done, go back to the first zone and the trainers should be refreshed.
Speed is pretty important for the ZA Royale, so quick battle starters like Aqua Jet and Extreme Speed should help. Personally, I like to use my Charizard's Hurricane move to reach opponents behind fences, pillars, etc. The move doesn't care if something is in between you and the other Pokemon: you just need to lock on for a second and use the move.
I hope this helps!
