RUBY AND EMERALD WEAPONS

I think you have to do a lot of work for these monsters. To tell the truth I defeated Sephiroth before these! I hope it helps! This guy says he knows a better way to defeat the Ruby Weapon? I haven't tried it.
These are the two most toughest battles in the entire game. If you want to take these beasts down, then you'll have to be well prepared. Here's a prep list for both of the battles that should make it quite a lot easier!
The essentials for an easy fight:
1). 'Knights Of Round' summon materia
2). 'MP Turbo' support materia
3). 'W-Summon' command materia
4). 'Mime command materia
5). 'Final Attack' support materia
6). 'Pheonix' summon materia
7). About 5-10 'Megalixirs'
It also helps if you've mastered all that lot!!! (You'll need at least 4 stars on your 'Knights Of Round' materia.)
Seeing as the Emerald (underwater) Weapon is slightly easier, go for that one first. Pair your 'Knights Of Round' materia with 'MP Turbo' (make sure the slots are joined) and equip 'W-Summon' on the same person. Then, on another person, equip the 'Pheonix' summon materia (from the top of 'Fort Condor' after the Huge materia quest) and pair it with the 'Final Attack' materia. (This is won from the Battle Square in the 'Gold Saucer' - You need to get 96,000 battle points and buy Cloud's 'Omnislash' limit break, and also the 'W-Summon' materia - incase you were wondering where that was. Then, talk to the lady at the desk and you can take part in a special battle where all the enemies are bosses! Win this and you'll be rewarded with the 'Final Attack' materia. If you're having trouble beating them, go and get some levels up (see my 'cheats and tips' section) or equip a 'ribbon' accessory.) This materia combination is extremely useful because every time the character with the 'Final Attack' materia is killed, he/she will summon 'Pheonix' and revive all party members!
Next, equip 'Mime' on the remaining person. Now you're ready to fight. Make sure you've got maximum (9999) HP and drive the sub up to him.
Although it's not essential, it might pay off to find the 'Guidebook' first. (In the underwater corridor just outside the underwater reactor, morph a ghost ship and you will receive this.) If you have the Guidebook, take it to the man in Kalm town and he will exchange it with you for an 'Underwater' independent materia. If you equip this, it will stop the 20 minute countdown.
The Weapon's attacks differ from about 3000 to 9999 (off everybody) HP. If it does 'Aire Tam Shot', you're buggered unless it 'misses' one or more of you allies, or you've got a barrier up. It is quite possible to get away without it ever doing this move, it only ever seems to do it if you let it spawn 'materia like' beads on its arms. The way to stop it doing this is to just keep dealing it excessive beats. Keep summoning Knights Of Round twice with your W-Summon and miming it straight after. Use a megalixir whenever you need to. It will die after 8 summonings! It has around 1,000,000 HP (and yes, that is a whole million).
You'll receive an 'Earth Harp' for defeating him. Where as this may appear to be completely useless, take it to the 'Kalm Traveller' in Kalm town, (upstairs floor of the house furthest to the right) and he'll trade it with you for the master magic, master summon and master command materia! (Nice).
Using your new materia, you can now challenge the Ruby Weapon. This will only appear after you have defeated Ultima Weapon. Look in the desert surrounding the Gold Saucer and you should see a small red tail sticking out of the sand, if you fly over this in Highwind, Ruby will jump out of the sand and attack you.
the tackticks for beating this weapon are different as it is slightly tougher, although it doesn't have quite as much HP. (around 800,000 I think). You'll find that attacking it straight away does not inflict any damage. This is becuase it only takes damage after it has stuck its arms into the ground and created two grey tentacles behind you. But before it does this move, it will try to suck away two of your opponents using its 'Whirlsand' technique. To stop it doing this, purposely kill two of your allies. that way, it won't suck away anybody and you can revive them after it has sprung its grey tentacles. Don't bother attacking its tentacles, just keep summoning Knights Of Round, but seeing as it counters with Ultima every time you do this, make sure everybody is at full heath before you cast it so they survive the beats. After it's cast 'Ultima' use a megalixir. (Useful having W-Item here because its tentacles cause abnormal status', so you can use a remedy and a megalixir at the same time.) On top of all this there's another drawback! Sometimes it casts Ultima twice in a row with no oppertunity to get a move in between. As Ultima does about 7000 points of damage to everybody, you'll need a Magic Barrier, and seeing as you never know when it's going to do this, you'll need it all the way through the battle. The Weapon will die after about 7 summonings. (Remember and make sure your Master Summon or Knights Of Round materia is paired with an MP Turbo).
When this has been defeated, you'll recieve the 'Desert Rose', which you can trade for a Gold Chocobo with the Kalm Traveller. (Pretty useless really seeing as it's almost impossible to kill ruby without Knights Of Round which you need a Gold Chocobo to get!)
Written by Dan Coupar
Thanks to Mike Blake