You can fight Emerald Weapon underwater after you have obtained a submarine in Part 2 of FFVII.
You can fight Ruby Weapon in the desert near Gold Saucer after you have fully defeated Ultimate Weapon.
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, you could turn the battle speed up to maximum under configuration in the menu...
Remember; the Calm with a K man wants the Guide Book, the Earth Harp, and the Desert Rose...
Don't be afraid to use Phoenix-Final Attack. When used, it will heal Ruby Weapon (Ruby Weapon absorbs Fire element), but only by at most 9999 HP. That may sound like a lot, but Ruby Weapon has around a million HP... the benefits of potentially reviving all 3 party members to full HP outweigh the fact that you would have healed Ruby Weapon too - Ruby Weapon, as I said, has around a million HP total; your party members have at most just under ten thousand HP each.
Have a browse around this website (hyperlinks at bottom of page) for more ideas on becoming powerful in Final Fantasy VII...
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Limit Breaks Guide
Enemy Skill Guide
Enlisting Yuffie and Vincent
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
Emerald Weapon & Ruby Weapon
About the Characters
Final Fantasy VII Remake Guide
List of Every FFVII Title
Other FFVII Websites