I suppose you could ditch Accuracy and level up Luck instead, if you have the patience. Maxed Defense would be quite good as well...

And for armor, I'd recommend: Auto-Haste, Defense +20%, Auto-Potion, Auto-Protect. Best for all characters.

Always kill the arms before you attack Penance him/itself and you shouldn't have any problems. For Auto-Potion to be effective, either get rid of all Potions and Hi-Potions and get 99 X-Potions or just get that result of Rikku's mix that causes all attack and heal to do 9999.