Here's how when you DON't want to go out of the way to level up your materia to master, or have 8 counters, or final attack, or whatever :

at lvl 70+, you can!! just luck. I found Emerald harder, get him first. Equip Cloud with KOTR, doesn't matter if it's not mastered, and Slash-All and give the other 2 people (mine were Vincent and Cid) only 2 materias (give one person HP plus and Mime and the other person HP Plus and Enemy Skill - you need BIG GUARD). The 2 MUST HAVE 9999 HP!! Then sit around and try to stay alive with Megalixirs - use W-item - (occasionally shaving off some of Emerald's 1 million HP with limit breaks and KOTR). When your unmastered KOtr runs out and you can't mime anymore, just attack. Get one guy to throw Megalixirs nearly every turn. MP's not that important anymore, just make sure your 2 extra people are at 9999 HP everytime it's Emerald's turn to attack. Then wait for Aire Tam Storm, if it hits your 2 characters at 9999 hp they'll be 7777 hp and get lucky 7 fever. THIS is where you'll get all that damage done. About 2x of that and he'll turn red and die. This is easier said than done because sometimes you might overshoot the Megalixirs and cure yourself from 7777 hp to 9999 hp back again (GUILTY!! ). But just be patient, even with just 1 or 2 lucky 7 runs you can finish the rest of it off with Omnislashes (and you'll be getting a lot of that). After which, run to shore and SAVE!!! Get your Master Materia please.

Now go to Ruby and all you have to do is equip Cloud with your newly mastered KOTR/MAster Summon, give your 2 other characters your 2 Mimes (you now have two - your normal Mime and your Master Command Mime). Give someone else the Hades materia/Master Summon if you didn't use that as well. Give your 3rd person Enemy Skill (BIG GUARD again). You must be able to cast KOtr and Hades multiple times, that's all the work you must prepare for this. Do the kill-the-other-two characters bit, ram the Highwind into Ruby. When the tentacles appear call Phoenix to bring everyone else back, then with your 2nd character (the one who has Hades) SUMMON HADES FIRST. This will paralyze him, muhahaha. After that you're FREE to cast KOTR without being countered with Ultima until the sky falls. Keep Big Guard up as needed, otherwise just call KOTR and Mime and Mime. But don't forget to KEEP TABS ON RUBY'S PARALYSIS, after about 6 KoTRs he'll come back to the world, just cast HAdes again.