
The North Crater - Jenova SYNTHESIS - and Sephiroth Himself. The final test of your strategy, nerve, and how many Turbo Ethers you've been able to hoard unless you're taking shameless advantage of the W-Item glitch. And...
Flew through it without breaking a sweat. :D
...yeah I might've done a bit too much grinding but I really wanted everyone to have a nearly-mastered HP Plus/MP Plus combo after all the TPKs I'd been eating and god damn if it didn't work. The Big Guard-Bad Breath-spam Beta combo carried me through to the final bosses, and then nearly-mastered Magic Plus+KotR pretty much wrecked Sephiroth's shit. For the final final battle, I let Sephiroth get a hit in so Cloud would counterattack, so even though I didn't fully adhere to 'no physical attacks' I did make good on 'no Limit Breaks'.
So, with the benefit of hindsight on this mad experiment, what have I learned?
1) As soon as you can, buy a second Lightning Materia - or at least anything but a second Fire. It's gonna make that cluster of Shinra Tower bosses + Motor Ball a lot less painful.
2) I never bothered with stuff like Sleep or Confu before, but they're a lot more useful in a magic-only run because magic doesn't snap the enemy out of it. Exploit the hell out of this. >D
3) On that note - Enemy Skill is going to be your best friend, especially late-game. High damage for relatively low MP cost, as well as access to useful stuff like Magic Hammer and Bad Breath. I basically had a party of blue mages and I don't regret a thing.
4) Unfortunately keeping up with weapon progression is still important, since your weapons affect your Magic stat. Which meant I was struggling for gil in parts, especially since some of the most useful Materia is eye-wateringly expensive. Idk, focus on your main party and try not to run from battles too much.
5) I did the Wutai sidequest for completion's sake but honestly idk if it's worth it for a magic-only run. It gives you access to some nice new weapons for that point in the game, but if you haven't recruited Yuffie yet then the weapon shop doesn't close, and you just have to wait until Disc 3 to go get Leviathan. The only Materia you get that you might not be able to otherwise is HP Absorb, which... y'know, ymmv but I never found that one very useful. Godo's fight in the Pagoda gives you a second chance to learn Trine, but that's about it.
6) ...but definitely get Yuffie, she and Vincent have the highest magic stats after Aeris. They're glass cannons, naturally, but for what it's worth they were my team in the North Crater.
7) Speaking of, there's no reason not to use Tranquilizers to induce Sadness on everyone, just for that extra boost in damage reduction.
8) Watch. Your. Max. HP. Learn from my mistakes.
And none of this is going to help me in my next run - I usually replay every couple of years, and next time I get bit by the FF7 bug I kind of fancy going No Materia. :D