BOSS ANALYSIS : Penance is, in essence, the strongest boss you’ll ever fight in Final Fantasy X. While it is not the strongest possible, with stats nearing their maximums of 255 in most stats, Penance will NOT let you get by with anything short of 255 in all stats except - maybe - Luck. Even at maximum Defenses, Penance’s attacks can still pierce 10,000+ damage without buffing. More on that kind of stuff in a moment.
Anyhow, the main body of Penance is clearly the main threat. It knows three moves. There’s Obliteration for physical damage to everyone and Slow. There’s also Immolation for physical damage to everyone, an Osmose effect (MP loss for you), and Armor and Mental Breaks (halved Defenses). Finally, there’s Judgment Day, his most volatile move, but only used after taking 3,000,000+ damage (
The arms also get their own moves. Both are able to use a basic physical attack, although the effects differ upon the arm. The Left Arm will be able to Dispel all of your positive statuses (and ONLY positive - can’t get rid of the Immolation’s Breaks this way), and this includes Auto-Life. The Right Arm can Petrify you instead. The Left Arm can also use Mega-Graviton; in this instance, it rips off 75% of your maximum HP and doles out Silence, Darkness, Sleep, Slow, and Doom (5 turn). The Left Arm can use Slowga as well for the obvious effects. All of the Left Arm’s moves seem to have an inherent Quick Hit/Pockets-like side effect of boosted recovery rates as well.
The Right Arm has attacks other than the basic Petrifying physical and, like the Left Arm, they also have a Quick side-effect. Calamity should be familiar; this hits you with Curse, Darkness, Silence, Poison, and all four statistical breaks.
The largest danger is Mighty Guard, really, used by the Right Arm - it will put Protect, Shell, and Regen on all of Penance, essentially halving all damage they take and restoring lots of HP for ten turns, based on the CTB’s tick system. The "tick" system is hard to describe, but let’s just say it does work on percentages of the max HP. It’s a lot here, too - one "tick" equals 46,975 HP of recovery, and there’s almost always 9 ticks between Penance’s turns, so we’re talking about 422,775 HP of recovery per turn for Penance for ten turns, which equates to just over a 35% recovery rate if left alone. Granted, all this was mathematical extrapolation from known formulas; I actually would never let him sit there even a single turn before a Dispel, just knowing the math.
MAIN STRATEGY ( 12,000,000 -> 9,000,000 HP): Before the battle, there are several abilities you’ll want on your armor, mostly if you’re playing without breaking 9,999 HP as I recommend. Ribbon is vital here to deal with a ton of the ailments you’ll be pummeled with. Also important is Auto-Protect, as it cannot be Dispelled and halves most of the damage you’ll take. Auto-Haste is essential to keep up with this guy as you’ll be outsped otherwise. And, finally, it comes down to Auto-Potion with an X-Potion-only stock, Auto-Phoenix, or Defense +20%. Yes, Defense +20% has its uses, since, coupled with Auto-Protect, you’ll only take 40% of the damage you’d normally take. It’s up to you there. All my armors had Ribbon, Auto-Haste, Auto-Protect, and Auto-Potion.
Without exception, you will want to begin by Mix-buffing. If you have the auto-abilities named previously, all you should do is open with Ultra NulAll. As usual, this is for the 5 Cheers, Reflexes, Focuses, and Aims; despite the lack of need to nullify elements, this will exceed your 255 limit on stats, bringing them up by about 10% from there (+25). This makes most attacks hit for under 9,000 damage with Auto-Protect easily enough, meaning Auto-X-Potion should suffice for curing, or Full Life under One MP Cost.
Your priority is NOT a direct offense against Penance; for the most part, that will just get you killed. While being under 10,000 HP is recommended for that extra armor slot, extreme care must be taken to keep your item stocks high, after all.
During the battle, after buffing, you’ll always want to focus on attacking the Arms, and specifically the Left Arm first; Mega-Graviton will be deadly, probably. Do this via Quick Hits, and you should have enough time to take out the Right Arm before anyone gets a turn. The point of attacking the arms first is to - other than to grind up Master Spheres =P - is to prevent the unpredictable Judgment Day that could result anyways.
Even if you didn’t get rid of the Right Arm, don’t worry. Penance itself will use Obliteration, which is mostly damage. Since the Arms’ Agility is lower, so you can go ahead 'n' Quick Hit the Right Arm to death before it gets in its attack. Heal whatever you need top after, then blast Penance’s main body after. If you can, this is a GREAT time to throw Blitz Ace at Penance; otherwise, more and more Quick Hits. Doublecasting some kind of magic (Flare/Ultima) also is a bit viable since magic is generally 20% more powerful here versus Penance, though the strength is equal versus the arms (hence Quick Hit then - do whatever you want here).
Loop as needed.
MAIN STRATEGY (9,000,000 -> 0 HP): You’ll notice that this part begins when Penance’s lower half falls off. It’s pretty easy to notice. =P Anyhow, Penance will open this in the Haste status and that’s NOT good; we’re talking 510 Agility, really. Hit 'im with a Purifying Salt or Dispel to get rid of it (Purifying Salts do damage - every bit counts!).
During this phase, you’ll begin to see Immolation coming from Penance; expect this to take off a chunk of HP and MP, so you’ll probably want to hit up with Megalixir (via Quick Pockets, of course) unless a fair amount of MP is left, like 50~100 or something like that, since you SHOULD have One MP Cost. Also be sure to hit the afflicted with Dispel at some point to get rid of the Breaks.
Beyond that, really, it’s the same as before - Quick Hit the arms, heal before Penance moves, and then Quick Hit/Doublecast Penance. Hopefully you’ll never see Judgment Day, which - with 255 Defense + 50% from Auto-Protect + 25 from Cheer x5 - will be the only legitimate threat.