ULTIMATE ARMOR
(my recommendations)
Auto-med
Auto-haste
Auto-phoenix
And break hp limit
For auto-med you need remedy - Rin on the airship sells them. You need 20.
BRIBES
Chocobo Wings - ABILITY - AUTO HASTE(80) - bribe machea in omega ruins - 360,000 gives app. 60
Mega Phoenix - ABILITY - AUTO PHOENIX (20) -bribe ghost in sunken cave - 200,000 gives app. 27
Wings to discovery - ABILITY - BREAK HP LIMIT (30) - TRIPLE AP (50) -
bribe malboro - in the calm lands - 560,000 - app.5 -
bribe great malboro in inside sin - 1,290,000 app.7 (I don't think great malboro is worth the trouble)
You can also fight Shinryu, but this is an underwater fight so you will only be able to use Rikku, Wakka, & Tidus for it. He drops 2 Wings to Discovery.
Confuseproof is also very useful, but you usually only need it for certain fights. You can steal musk from floating death in omega ruins. It takes 48 musk to get confuseproof. I�m not sure who to bribe. I also think Ward�s are just a waste of your space.
Auto-Regen will just be a waste of space once you have break hp limit, but you can easily get it from fighting kottos a couple of times.
I have the US version so I don�t have ribbon, it�s been said the ribbon is better than auto-med.
You can buy weapons with 4 empty slots from Wantz in Macalania Woods - South, very expensive 100,000 each. This is probably after you have the airship.
I wouldn't start fighting the monsters the arena guy makes until after you have a couple of characters with 255 strength and Auto-Phoenix. Those two things will probably be your biggest help.