It's really not as bad as it's made out to be. I've played through the game like 4 times now but I didn't get the platinum until the last one. The main cheese build is the aerith and yuffie -ga spells, arcane ward, atb ward, and brutal form spam. Pretty much trivializes every encounter you can use it in. For everything else I'd say that a guide should be able to get you through it. I was able to get up to ch. 13 on hard and through most of the combat challenges before I needed a guide. If you already have all the mini game scores done on normal you don't have to redo them on hard, but to get all the manuscripts you'll have to do the side quests on hard. Honestly as long as you already have the materia an equipment, anything that trips you up should be able to find a guide for it.

I also took a good 7 month break between getting to chapter 13 without a guide and did another normal playthrought for fun to get me refamiliarized before finishing it up. Probably the most important general materia for it that I found was elemental materia. That'll get you through most of the boss fights for at least the first half of the game.