TrueSaikred's Method is great but theres a less AP grindey method I stumbled upon across the internet
Kill 2 party members.
Equip the remaining member with:
KotR (Any Level) <-> HP Absorb (Any Level)
Mime (Any Level) <-> Counter (Mastered)
W-Summon (Optional)
A couple of HP Plus materia
Then:
Engage Ruby.
Wait for tentacle plant.
Cast KotR.
AFK
After you cast KotR, Ruby will counterattack with Ultima. Then you will counter with Mime, recasting KotR. Then Ruby will counterattack with Ultima. You’ll counter with Mime. Each time, KotR will fully heal you with HP Absorb.
About 20 minutes later, Ruby WEAPON will be dead and you will have cast KotR between 8 and 12 times, depending on how much damage each cast does.
Credit:
https://www.quora.com/Final-Fantasy-7-How-many-times-can-you...