Since we're playing a FF game:

1. Exploiting weaknesses, from elemental or status - If we have a Pieste boss, why can't blind prevent gaze attacks?
2. Bosses that get desperate to kill you before you kill it - Once it hits 20% or lower, that's when it should be a desperate struggle to survive. I mean, you're TRYING TO KILL IT.
3. Requires every party member to know their job, no carrying and gliding through.

Dungeon wise, something open world where you can see other people trying to get through the depths with the boss area being the actual instance.