If u have these abilities attached to your armours: Auto-Haste, Auto-Protect, Def +20 and Auto-Potion, then you stand a fighting chance against Penance even w/out Break HP Limit. Just make sure no arms get a turn by executing the Quick Hit command, as this armour is not setup to properly defend against them.
Def +20 should suffice in lessening the dmg dealt upon you by Obliteration to less than 9999 for the time being. As long as there’s a stock of potions that’s solely X-Potions, the Auto-Potion ability will automatically heal you to full-health every time you get hit w/out wasting a turn. However, Def+20 and Auto-Protect won’t be enough to protect your characters from Immolation later on. This is where Rikku’s mix is gonna save you. Make sure you carry several Light Curtains and Healing Springs on you as these are the ingredients for the Ultra Nul All mix. Immolation can also inflict Full Break on your characters, so Dispel the effects before he performs it again.