It is possible to fight Ultimate Weapon near the end of Disc 2 or in Disc 3.
You can fight Emerald Weapon underwater after you have obtained a submarine in Disc 2 of FFVII.
You can fight Ruby Weapon in the desert near Gold Saucer after you have fully defeated Ultimate Weapon.
He will appear hovering above a newly created water terrain near Junon. Fly at him with the airship, and a battle will be initiated. After winning this fight, you must chase him and ram him over and over again with the Highwind, until he comes to rest and you can fight him again. After defeating him several times, he will hover near Cosmo Canyon. Defeat him here and he will be completely finished off, leaving you significant amounts of EXP and AP, as well as Cloud's ultimate weapon, Ultima Weapon.
The fights against Ultimate Weapon are relatively easy. You can breeze them simply by having powerful materia such as Double Cut, Quadra Magic, HP Plus etc.
It is highly advisable to steal each time you face Ultimate Weapon. You can obtain powerful Circlet and Reflect Ring accessories.
There is more than one technique for defeating Emerald and/or Ruby Weapon.
If possible, turn on God Mode (press R3).
Useful materia: W-Item, other powerful materia.
An effective technique is to attack with one turn, use a Megalixir next turn, then attack, then use another Megalixir, then attack, then use another Megalixir, etc. etc.
You can create Megalixirs with the W-Item cheat.
Some powerful attacks you can use are e.g. Knights of Round, 4x-Cut.
As a "safety measure", you can equip Revive-Final Attack or Phoenix-Final Attack materia.
HP Plus materia can also be useful...
Rather than morphing a Ghost Ship to obtain the Guide Book to obtain the Underwater materia to remove the 20 minute time limit, you could turn the battle speed up to maximum under configuration in the menu to make it easier to win the battle within 20 minutes...
It is advisable to deliberately enter the battle against Ruby Weapon with only Cloud alive. If not, Ruby Weapon will remove 2 of your party members near the start of the battle with a whirldwind type attack. By deliberately KOing the other 2 party members, you can ensure you will have Cloud in use for the fight.
Ruby Weapon is quite susceptible to [Stop] status. You can take advantage of this by using materia which can inflict [Stop], such as Choco-Mog, Hades, Freeze spell on Contain materia, or the actual Stop spell from Time materia.
Finally, just use a similar powerful setup as against Emerald Weapon; use Double Cut, Knights of Round, Revive-Final Attack, HP Plus etc. Hero Drink items can also help.
Remember; the "Kalm traveler" (man upstairs in one of the houses in Kalm village) wants the Guide Book, the Earth Harp, and the Desert Rose... he will give you a Master Magic, Master Summon and Master Command materia in exchange for the Earth Harp (which you can win by defeating Emerald Weapon); a gold chocobo in exchange for the Desert Rose (winnable from Ruby Weapon); and Underwater materia for the Guide Book (morph a Ghost Ship either in Junon Underwater Reactor or in the Battle Square in the Gold Saucer).
Have a browse around this website (hyperlinks at bottom of page) for more ideas on becoming powerful in Final Fantasy VII...
My FFVII YouTube Channel - PLEASE SUBSCRIBE!
Limit Breaks Guide
Enemy Skill Guide
Items Winnable from Battles Guide
Enlisting Yuffie and Vincent
Status Effects Explanations
Wall Market Cross-Dressing Guide
Turtle's Paradise Flyers Locations
Shinra Building Floor 62
Shinra Building Floor 63
Junon Parade Guide
Gold Saucer Guide
Huge Materia Rocket Code
Alexander & Added Cut Materia
Materia Selling Guide (make loads of Gil)
Ultimate Weapon, Emerald Weapon & Ruby Weapon
About the Characters
List of Every FFVII Title
Other FFVII Websites