I'm skipping around in Hard Mode right now. To farm and grind. There's no point grinding in Normal past a certain point because hard mode is like 2x EXP and 3x AP - and there's EXP and AP materia to get as well.
BTW, the white dog sidequest from the sector 7 slums is WAY harder than the Scorpion boss in the Mako Reactor.
Hard mode means you just have to prepare for each section and boss fight. Like no surprises, this is the shyt you HAVE to do, preparation. Someone else mentioned First Strike - its whatever in Normal. In Hard Mode its a necessity because it means you can just get right into what you need to do to kill enemies early or get defenses up fast enough to not get bodied.
Enemy Skill is extremely important. Just too versatile for a single slot.
HP up is as good as you can get to upping defensive stats substantially.
Chakra and Pray ofc. Revive is essential. I actually like Bio as well because it won't ever miss and its cheap
I'll think of some more later but those are kinda must haves imo. Things change a lot defending on who your group is.You cannot do Barret healer-tank shyt with anyone else.
I'd definitely give manawall + magnify/lifesaver a shot. Should fix all your problems. Make sure you've got Pray on Barret as well. Maybe Chakra on both since its a two man battle.
I'm not sure if you can slow that boss, but I'd definitely recommend people start looking throwing side effects on bosses when given the chance too.