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...
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
My YouTube Channel. Please SUBSCRIBE!
Final Fantasy VII Remake Guide
List of Every FFVII Title
Other FFVII Websites