It turns out that Dark Yojimbo hides one of his 5 fights down a dead-end. I checked that dead-end this time through the sealed fayth cave and was able to unlock Penance. I saved and went to give him a shot to see what I was up against. I got obliterated! Penance fired off an attack that hit me for ~80% of my health on all 3 characters and slowed Rikku. He also has 2 arms that do their own attacks, one of which attacked everyone for ~70% of their max health. I hadn't had a chance to heal up yet, so I was dead.
Ok, try again, this time without wasting time hasting Rikku. I used some defensive stuff and decided to try to burn down an arm by using a bunch of overdrives. It worked! Woo! But the other arm and Penance himself burned down the Magus Sisters that I'd busted out to overdrive. And then the arm I killed came back. I threw everything I had at it, and it came right back!
Clearly I was not strong enough to win this fight. Rikku getting slowed is too big of an issue. With the arm respawning so quickly after death this fight is going to be a DPS race and not having 3 hasted characters is game losing. So I'm going to need to build her a different suit of armour that has auto-haste on it. Oh, and none of my people could even hit any of the arms. So now that I've killed almost all of the challenge bosses that require high luck I should probably grind up a bunch of luck. Because I don't think I have time to bust out 15 uses of stat buffs in order to start killing arms.
Killing all those dark aeons actually got me a lot of master spheres which can be use to activate luck nodes. So I mostly just needed to grind up luck spheres, not luck spheres and fortune spheres. I was so close to a full sphere grid achievements so I just went and finished that off. Mostly with luck, but also with some more health. I bribed up the materials to add auto-haste to a suit of armour for Rikku (I ended up going with break hp limit, auto-haste, and auto-phoenix which was not optimal). I also looked up a list of Rikku mixes so I could actually use her limit break effectively. (One of the mixes actually gives 5 uses of aim and cheer! So for the time it would take to put up 3 buffs I could do 10.) Another mix restored everyone to max health which unfortunately isn't the case for a normal elixir. Elixirs only restore 9999 health which isn't nearly close to max when I have 620000 max health!
I also finally used some of the consumable buff items. In particular I used one that doubles my max health and one that made all my spells and abilities cost 0 mana. The second one was particularly relevant because Penance has a second form that drains all of your mana with every attack.
I did a little bit of reading on the fight and the wiki said every 19 times Penance attacks or that you attack Penance he'll cast an attack that is guaranteed to kill your party. But it doesn't dispel auto-life, so I can just put that up and hope to recover with some Rikku mixes. I did the fight a couple times learning the patterns of things and found it wasn't so bad. With 5 stacks of aim on and the luck I grinded I could reliably hit the arms all the time with Tidus and Yuna and most of the time with Rikku. It only took 6 quick hits to kill each arm and if I killed the arms right away I would have enough time for 3 actions before they respawned. Mostly that could be attacking Penance, sometimes it would have to be healing up from his attack. I have plenty of megalixirs to burn so healing up from his phase 1 attack wasn't so bad.
Note that Penance has 12 million health and I attack for 100k per attack. So if I average 2 hits per cycle I need to pull off 60 cycles to kill him. That's a long time, but it's at least plausible.
Then I hit phase 2 which had him change what attack he uses. Now instead of hitting my team for 7k each he'd hit one person for 10k and inflict a bunch of breaks on them. These breaks meant that the next time he hit the person they'd be killed. Dying is bad because I need those aim and cheer stacks and even if I had infinite overdrive power I wouldn't do any damage if I had to mix those buffs up every 2 rounds. You can remove the breaks by casting dispel on your own character, which is good. But that also takes out protect, which is bad. Without protect his attack hits for 20k and suddenly I'm stuck taking 3 actions per cycle to counter his attack. I only have 3 free actions per cycle and can no longer make progress.
Ok... I can actually generate more time per cycle if I delay killing off the arms. Final Fantasy X gives you the exact turn order for each actor in a fight. I can save the 6th hit on an arm for immediately before it would get to attack! Since it seemed to take the same amount of time to respawn the longer I can keep it alive (without letting it attack since its actions were brutal) the longer it will take to respawn. This let me stretch things out such that I'd get more like 7 free actions per cycle that weren't dedicated to killing arms. It did mean the boss would get more actions per cycle too, but it was a net gain for me. Especially since if I could set up a situation where all of my party was injured I could condense some of my healing actions together by using megalixir or Rikku's full party heal mix. I don't know if there's a window to recover from that super attack thing, but on my attempt after the one where I figured out to build longer cycles I decided to count out every single attack so I could time it out...
As an aside, this fight really reminded me of raiding in World of Warcraft. A fight that initially seemed insane but once you got the gear to overcome the DPS check and a timer to time out the powerful abilities you could work out a strategy and then try to implement it perfectly. I do miss that.
Anyway, fight away... I wasn't sure what was going to count as an attack or not so I counted things out separately. I hit Penance 32 times before he changed form, and then I hit him 47 times before he used his super move. In that time he'd used his immolation ability 23 times, I'd counter attacked him 7 times, and I'd used one purifying salt on him to dispel his haste. That seems like a lot more than the 19 I was promised! But considering I'd done more than 8 million damage it would probably be ok, assuming I could recover from all my characters being dead. It was truly terrible timing for me too, since his second arm had just respawned. Dying and coming back to life with auto-life seems to add a delay to your characters too, so his arms got to do their things right after my party died. On the plus side they used single target attacks on Yuna but she got right back up thanks to the auto-phoenix ability on my armour. It was tense to work out how to recover (I had to pop 2 megalixirs and let everything live another round in order to have the health to survive a combo arm AE attack + immolate, but the other arm wouldn't go until I got to go again and finish it off), but I did it! One of the great things about the FFX combat system is you can actually take the time to think through a plan given the turn order. I managed to stabilize and get back to hurting the boss.
And then it all came crashing down. I hit him 13 times, he hit me 12 times, I countered him 5 times. He was now under 2M health left. But he used his kill everyone ability, again with terrible timing right after the second arm had respawned, and this time I couldn't recover. Because the arm didn't autoattack Yuna, it autoattacked Rikku, and apparently it has a petrify component to it. Tidus and Yuna have ribbons on so they can't get stoned but Rikku could. And in this game if an attack both stones and kills you then you're permanently removed from the fight. In a DPS race situation, losing one member permanently is the end of it. I decided to try to burn the boss down because there was no other chance but it turns out Anima's overdrive does less damage than an autoattack. I'm sure I got him under a million and it's possible if I'd stopped attacking arms entirely that the extra 700k damage I wasted on them at the end was the difference.
Blerg. The fight lasted an hour and thirteen minutes! And I didn't win!
Ok, do I need to farm Rikku a ribbon? And what's the deal with that ultimate ability anyway? I tried summoning out an aeon to block it when I hit a 19 count near the end of the fight but he didn't use it and it just wasted time to bring it out. If I could get an actual counter on when it would happen then I could survive it easily. But even with a ribbon on Rikku it's no guarantee that I'd be able to recover from a single use of the ability. I had to get lucky to not have an arm use an area attack and kill me off instantly as it was!
So I turned to the internet to see if I could find a timer or something. And found someone who posted that he only uses the super powerful attack if the counter is at least 19 AND both his arms are alive. So it wasn't bad timing that caused him to use it twice when both arms were alive... It was a game mechanic. If I could have just kept going without him getting a turn with 2 arms alive I would have won. And since it turns out the amount of time it takes for an arm to get an action is exactly the same as the amount of time it takes for a dead arm to respawn I could manipulate the setup such that every time I killed one arm the other would respawn. I could make it so, if I didn't make a mistake, the entire fight would eventually take place with exactly one arm alive.
I went to sleep, got up, and got to work. I changed how my overdrive was charging just in case I needed to use extra mixes (I was on 'charge when you take damage' and switched to 'charge when you deal damage') and set to work. I screwed up counting a little at the start because I was still a little sleepy but once I got into the groove of things it was 'easy'. Just play perfectly for a little over an hour! No problem!
So that pretty much puts a cap on that. Time to beat the game I guess.