beginning tutorials

[sub zero]

for diablos you need selphie zell and squall. have squall with about 3-4k of hp + attack rating about 100 power should do.
wait till it dose gravija use zells duel and squalls reszuken and have selphie cast full cure useing the slots limit rinse and repeat. you might wanna have "WAIT" battle mode on.

[TheSpoonyBard]

For Diablos just Draw and Stock Demi spells, then cast the ones in your stockpile. Repeatedly doing so will bring Diablos' HP down low enough to be finished off with physical attacks. Also, whenever Diablos is hit by a Stocked Demi he will cast Curaga on the characters, so you'll always be at full HP. Blind can help reduce damage from his physical attacks if you're having trouble drawing Demi spells from him.