Required:
Morph materia
Being on disc/part 3

Recommended:
Being able to consistantly hit for 9999 damage to easier judge enemies health remaining
Yuffie's Conformer weapon to counter the low damage attacking with Morph would give

Location:
Northern Crater swamp area
(encounter is possible in other areas but felt like highest rate here)











Encounter:
Master Tonberry

• HP: 44,444
  
• Immunities: Gravity, Death, Slow numb, Berserk, Petrify, Sleep, Confuse, Frog, Silence, Small, Manipulate
  
• Susceptible to: Poison, Paralysis, Stop
  
• Attacks: Everyones Grudge (only used if attacked 3 seperate times before he takes a step, deals damage equal to total kills made by character times ten)
  Knife (only used when close enough to cause instant death)

Quickest strategy:

• Using a levelled 4x cut for 9999 damage per cut (or 4 standard 9999 attacks)
  
• Yuffie using the Conformer to max damage (or at least 4448 dmg) Morph the Master Tonberry into a ribbon
  
• Repeat until you have enough ribbons.

Less efficient strategy:
If you cannot reliably do 9999 damage per attack or close enough to make the prior strategy viable and/or do not have Yuffies Conformer then a bit of math may be necessary or a lot of trial and error casting Morph, doing it like this can take much longer but by exploiting his weakness to stop to give you more time can be a lifesaver while you try to whittle his health down low enough to morph.
Suggested materia: Added effect linked to Choco/Mog on weapon to inflict stop on your attacks.