Equip party members with Mystile Armor (Definitely if you dont plan on using Hades against Ruby)
Use Big Guard Enemy Skill (BG)
Final Attack-Phoenix on your tankest (most HP) character
Preferably link KOTR with mp/hp absorb and/or mp turbo. You can also link a Master Summon to one of these for same effect.
If either of the weapons use Status effects (I believe Rubys claws can) than Ribbon is an amazing accessory to equip
Ruby Trick: Go into battle with 2 characters dead, Ruby will stick its claws into the ground, you can then revive or summon Phoenix (to revive)
and have 3 chars for this fight
Note: Hades (Summon) / Dazers (Item) can paralyze Ruby (May want to try a w-summon Hades/KOTR for a safer but longer Ruby Battle)
Note: Having Underwater materia equipped gets rid of timer on Emerald
Note: Gravity will do 9999 dmg to Emerald (Quad Magic+hp/mp absorb linked can be a solid assault) <- Atleast until Emerald is VERY close to dead
Note: Aire tam (notice, materia backwords) Storm will do damage depending on how many materia your character has (1111 x # of materia equipped iirc) <- I usually just
counter(ignore) this with final attack + phoenix.
Note: You can duplicate Megalixirs with the W-Item trick if you want.
Note: If you link the same materia to separate support (blue) materia, all the effects linked will be activated in battle.
Example of final note: You have 2 Fire Materias, one linked with HP Absorb, the other with All, when you cast fire it will hit all enemies AND absorb HP.
This can also be done with the Master Magic/Master Summon Materia. A common trick is to link Master Summon with HP Absorb and KOTR summon with MP Turbo, giving a max damage KOTR where each hit absorbs HP
Open with BG
Cast W-summon KOTR (preferably linked with hp/mp absorb)
Mime until you die
FA+Phoenix will revive you
Mime until you are close to dead
use a megalixir
Alternatively, you can W-magic a quadra magic demi (with hp/mp absorb) and when it stops doing 9999 dmg use all your limit breaks to finish it.
Again megalixir is your best bet when weak.
Go into battle with 2 chars dead, and the third with FA+Phoenix.
When Ruby puts claws into ground, revive team.
W-Summon KOTRx2 or Hades+KOTR (RECOMMENDED)
Mime until you are near dead
If this Ruby strategy does not work for you i can edit with a separate one.
Remember both these monsters have insane HP so these battles will take a while, even with the ridiculousness that is KOTR.
I also prefer to equip Scimitar and Apocalypse because Ruby and Emerald give insane AP (50 000 iirc) And these are the only 2 weapons with Triple Growth slots
Hope this helps, Cheers