What changes
The chapters that hurt
Chapter 11
Chapter 11 has five bosses — the Diabolic Variant in the reactor, three in the manor and the facility, Roche outside in Nibelheim — and on hard no bench between them refills MP unless you spend a cushion there. Build for MP: Prayer and Chakra on everyone, Barrier over Cure, and the Elemental-on-armour trick for the Diabolic Variant and Yin and Yang.
Chapter 13
The temple splits the party into groups you did not choose, so all seven need hard-mode-ready loadouts. Keep an MP Absorption pair on each caster.
Chapter 14
One long fight in phases with every character. Bank limits, bank synergy, and go in at full MP — there is nothing else to spend it on.
Chapter 10
Barret and Red XIII alone in the Trials of the Gi, against Gi Nattak. No items: Barret heals with Prayer and Red XIII's Vengeance heals him, and Lifesaver is best avoided here, as it drains Barret's MP.