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Lock.. for Emerald there is a little bit of a strategy.

You need to have everyone with mime (if possible) if not.. then the one person without mime needs to hold W-Summon + Bahamut. Then of course one with KOTR + HP Absorb. Then lastly someone needs to have Final Attack + Phoenix.
No other materias PERIOD! The thing that kills everyone is the Aire Tam Storm. Basically the more materia you have.. the more thorough of a raping your going to recieve.
You start out... and KOTR. And keep miming the hell out of it. And if one of your people doesn't have Mime... then you need to keep skipping their turn. (DON'T DEFEND OR SWITCH.. just hit Triangle. Itll go to the next person.) Eventually its going to be inevitable... that someones going to get offed. And thats fine. So keep on raping it with KOTR. When the person with Final Attack gets offed.. then W-Summon Bahamut. You may get a chance to do it once or twice. Then return to KOTR and continue raping. Its a bit of a b***h.. and may take 2 - 3 tries. But you'll get that lucky break.
Also the boots that give auto-haste = love.

How've you been Mako? : P

But yeah... Emerald = hard as bloody hell. If all the bosses in the game were just slightly less tougher than that... i woulda been happy. Because i like it when you actually have to bloody think when your fighting. Not just Attack, Fire, Attack, Fire.

sad to say Emerald is my hardest.... i even had the underwater materia. never got past the first two hits. ever. but then again i'm going to replay and find everything then try to kill it. Ruby i think i almost had him dead once. omega. pff i beat him all he time. just try to avoid shadow flare and i'm good

btw i wasnt hacking you can morph every enemy in the gelnika into a source and all sources are available there get yuffies str to 255 give her morph and i think the all combination it may have been mega all and you can get your full party to all max stats in someodd like i think it was 30 or so hours and in the process the gelnika gives the best materia growth rate sooo

you practically have to be a god to beat ruby with knights of the round, at least with the version released in the us he one shots you in response to every cast of knights of the round with ultima, w-summon=2 ultima counters if you dont have mastered final attack-revive combo and fully maxed stats as well as gear built for magic evasion which i fought him with hell withstand about 8-10 knights summons and counter with ultima every one of them killing you each time again unless you have MASSIVE magic evasion it is not recommended to use knights even once against him, which is why he was so tough for me though i still finished him with kotr i hit him with it several times and luckily evaded all but 2 of his ultimas still for me emerald was a cake walk in comparison

generally reccommended summon for him is w-summon quad magic zero buhamut he wont counter that but it makes the battle longer just hopefully longer with you alive not creamed by ultima spam

Wow.....I just finished reading all the answers, and they remind me way too much of the strategy guide. (And none of you mention Enemy Skills......why not? It gives you access to the most powerful spells for cheap. Take Angel whisper, It's a full heal, Esuna, and Full life all wrapped up into one. And it only costs 90 MP (Maybe 100, can't remember. Been years since I had the game) For any beginners, grab the enemy skill materias and collect as many as you can. They give you a huge variety of spells (Healing, Attack, and Special) and don't do anything to your health or defense (Mind you, they don't raise your magic and MP either)

emerald was gettin on mi nerves scream scream scream