I've finally figured out what makes Hard Mode battles bulltrout.

It's the sudden and sharp decline in AI performance. In Normal Mode, your party is clever enough to block and dodge away from attacks and will straight run to the far corners of the arena when a big attack is called out. In Hard Mode, your fellow party members become f-ing retarded, rushing at the enemy during its deadliest attacks.

Hard Mode isn't hard because the bosses are badder. It's hard because your team is stupider.