Defeating the WEAPONs
The WEAPONs are by far the hardest creatures of the game... even harder then both of Sephiroth's forms and Jenova put together. I was able to take down the Ultimate WEAPON rather easily, no higher than level 60, but dont even try the Emerald and Ruby WEAPONs without some major materia and levelage. Diamond WEAPON isn't very hard, but that is because you fight him near the end of disk 2! Emerald and Ruby were added by the American team, not appearing in the Japenese version (or any of the game's movies) but that makes them no less challenging. Beating them, however, gives you three things: Pride, pleasure, and most importantly, REALLY COOL STUFF.
The Ultimate WEAPON
Contrary to his name, the Ultimate WEAPON is actually the easiest of the three beasts. The hardest part is chasing him around the map and wearing him down. Right when you start the battle, cast Regen to keep your life up. Then pound away with your top spells, like Ultima, Bahamut ZERO (quadra'd if you have it) and Knights of the Round. When you deal enough damage to take out a small army, WEAPON will run away. Chase him in the Highwind until he hovers over another town, and attack again. He will typically head towards these areas: The Crater-lake near Junon (initial appearance), Nibelheim, Mideel, Midgar and Cosmo Canyon. When he hovers over Cosmo Canyon, you almost have him beat, because he can't heal between battles. Slaughter the beast, and he will fall quite satisfyingly to the ground. Once he falls, Cloud recieves his ultimate weapon: the Ultima Weapon, and a path into the ancient forest opens up. One note: Have one (or all) of your characters equipped with the Enemy Skill materia. Ultimate WEAPON has a final attack of Shadow Flare, and it is a very potent spell to learn. Sephiroth uses this spell, and it can be devastating.
I really, really hate this guy! He has a good 500,000 hit points, and some of the meanest attacks you can think up. On top of that, there is a 20 minute timer to beat him! This last problem can be remedied by getting the Underwater materia from the Kalm Traveler. Just morph a Ghost ship on the way to the underwater Reactor to get a guide book, and give it to the Traveller. You recieve the Underwater materia in return. Equip the Final Attack-Revive or Phoenix materia, and all your other powerhouses including W-Summon, Knights of the Round, and Mimes on all the characters. It takes 7 KoR to kill this guys, so have plenty of MP ready. At the beggining of the battle, he can deal 7,000 damage to one player with his laser and 3-5,000 with his stomp attack. Just keep healed (barrier helps a tad.) Once you deal a fair amount of damage, however, you are in trouble. His small lasers open up, and he can now deal to 8 attacks in quick succession. Half of these take away about 3,000 hit points, and the other drains about 3-500 MP. Keep plenty of Turbo Ethers, Megalixers and Regen about you. His worst weapon, however, is the Tai-aire storm (SP?) that deals 9999 damage to ALL players. If you have no final attack-revive, you are dead. Period.
There are several strategies suggested to take out this terror. The first is have all of your HPs at 9999, and in addition to Final Attack-Revive, equip Counter on every character. This way you can take advantadge of every attack, and use your own turns to heal. Megalixers work REALLY well in this situation. Also, you can just use W-Summon/KoR to cast two Knights of the Round, and use the other characters to mime it until Emmie bites the dust. Again, it takes about 7 Knights of the Round to kill it.
TIP SUPPLIED BY GYROS: Emmie is extremely weak to gravity, since he has so much life. Demi 1 will deal 9999 damage every time. Quadra'd and using W-Spell, this can be a potent way to supplement your other attacks. Don't rely on this too heavily, though, since it would take 50 regular Demi's to take Emmie down. For beating the Emerald WEAPON, you recieve the Earth Harp. This can be traded in Kalm for three master materia... good deal!
This baddie makes Emerald Weapon seem as wimpy as the oatmeal you had for breakfast. Dont take him on unless you are plenty strong, or plenty fast. He is found in the desert after you defeat the Ultimate WEAPON, shown as a talon sticking up from the sand. Run into it using the Highwind or a Gold Chocobo, and once again say a little prayer. You should really go into the battle with two members already dead, so that his hands are immediatly burried in the ground. Otherwise, he will be immune to your attacks, and he will use "whirlsand" to make you lose one of your members for the rest of the battle. Then just heal up, and maybe pray again. A good trick is to use W-summon Hades-Knights of the Round. Hades will paralyze him if you are lucky, and then KoR him. Then Mime this as much as possible (this is where the master summon materia you got from defeating Emerald WEAPON comes in handy.) Be careful, however, because if he isn't paralyzed he will probably counter with ultima. In that case, I hope you had some MBarriers and regens going... Also, the counter trick used against Emmie works well here too. Just stick in there, and hope he doesn't unbury his hands. If he does, just heal, and pray he doesn't use Whirlsand. Keep Final Attack-Revive handy too. Just hang in there, because I believe he has around 1,000,000 hit points. If you can keep him paralyzed, you should stand a fighting chance. Megalixers help a LOT in this case. After winning, you recieve a Desert Rose, for which you get a Great Gold Chocobo.