Even in casual mode, I had a fair share of difficult battles, like certain Rift Pokémon and certain major characters (almost rage quit on Angie & Cera, took a break from recording, and destroying them off-screen before continuing). While most of the AI trainers have Pokémon with 10 IVs and 85 EVs across the board, a few of them are fully EV trained. It should still be manageable, but give the opponent a chance to setup, and they can eventually sweep your entire team.