I haven't tried Expert, but Ultimate is harder. An ATK debuffed trash mob (top in first pull) did over 3.6k damage in a single triple attack on light element. That mob alone is dangerous. The boss's rage overdrive can wipe your entire group if you don't have ATK debuffs or DEF buffs active.
A good strategy is to reserve debuffs for right before an Overdrive cast. That gives you 2 guaranteed turns of active debuffs before they can be dispelled. Even better if you can time it for right before a Burst. I managed to mitigate the Rage Overdrive to about 2.5k damage using this strategy and a triple healer party (Cassiopeia, Sol, Nike). Even after eating two Rage Overdrives in a row my party was still fine.