I hope this isn't considered very bad form to suggest
but:
Seriously I would try using
yojimbo. If you can build a good relationship with him (see the FAQs on this) then you can kill Eartheater in a short time with minimal effort.
You can afford to pay him a decent sum (say between 5-15,000 gil) because you can often loot earth eater for items worth 50k, sometimes even 150k gil (or just keep the triple overdrive/auto-potion 3 slots if you like).
The only downside is you will never get an overkill, therefore only I fortune sphere. But without a long and painful battle, and no need to have an elite party.
Obviously I would not generally recommend this approach because it makes the whole monster arena pointless. But to get the extremely rare and important fortune spheres surely it makes sense. Why knock yourself out?
