FINAL FANTASY X - Penance Last Moments

For the third time, I've conquered you Penance. Now it's official, documented and registered on Steam. Evil Laughs

Not a difficult fight, though. Just time consuming. As long as you have a good strategy at the beginning with Rikku's Mixes, the arms will only get a turn at the beginning of the fight, you'll be able to manage Obliteration on his first phase, and Immolation on the second phase.

I don't remember the exact same order but basically, my strategy using Break HP Limit was:
0.9 - Everyone should initiate the fight with full Overdrive guage, and Comrade as their Overdrive Mode
1 - Rikku Mixes Hyper Mighty G
2 - Tidus Entrusts to Rikku
3 - Wakka uses Stamina Tonic on party
4 - Tidus starts to damage Penance's Left Arm with Quick Hit
5 - Wakka follows up with the same
6 - When Rikku gets a turn back, she Mixes Ultra NullAll
The arms will likely get a turn at this point, and the best outcome is for them to cast something your Ribbon and Auto-Haste will prevent like Calamity and Slowga. Worst possible outcome is a physical attack to kill one of your party members. Right Arm can cast Mighty Guard. We don't care to Dispel Penance, but we do care to dispel the arms. Our main concern is to set up our party and kill the arms.
Manage from there until Rikku get's Overdrive again to cast Trio of 9999 for healing with Al Bhed Potion, Mega-Potion and Potion (for second phase). You want to ALWAYS use Quick Pockets to use regular items.
Kill the arms ASAP, and try to get Overdrive to Rikku again.
When both arms are down, we want to use Three Stars on Party, and Mix Eccentric and Hot Spurs with Rikku.

From there, we continue with the left overs;
We dispel Mighty Guard from Penance (if Right Arm casted), we maintain our party healthy at full HP and we make sure Penance never gets a turn with the two arms up. Else, it will cast Judgement Day which dispels Auto-Life and hit for max damage (99.999), so it's a game over. Again, killing the arms, specially the Left Arm is number 1 priority. Left Arm can cast Mega-Gravition, that's why it's your main priority. Wakka's Attack Reels is essential to manage the Arms, so be sure to be consistent with getting 12 hits.
On second phase, Immolation causes All Breaks on whom it hits, so we gotta dispel it.
The rest is history.

That's the rough idea. I don't remember exactly how the turns go.
You don't need the other party members but Rikku, Tidus and Wakka.
Their stats were (Expert Grid):
HP: 30K~
MP: 999
STR 255
DEF 255
MAG 140~
MAG DF 255
AGL: 255
EVS: 255
ACR: 255
LCK: 240~

I never maxed MAG stats because it's definitely not needed. It's useless.
That's about the max MAG value you can have by activating all MAG nodes on the grid that the game makes available to you from default. MP you maxed out by just activating the default ones the game makes available to you as well. So those are two stats you definitely don't need to farm for.
Although I farmed for EVS, ACR and HP to max, it's arguable if you need them or not.
You don't need to break HP limit to win this fight or any Dark Aeon fight, but the strategy will be different.
Dark Aeons, aside from Ixion, will hit you with their normal physical attack regardless if you have Max EVS, so you don't really need to form for it either.
Even if you have Max ACR, you can't hit most of the Dark Aeons, your attacks will miss most of the times. In order to hit them, you need to have 100 more luck* than the enemy.
*I've seen people say it's 75, but I rather be safe.
Dark Mindy, Dark Bahamut and Dark Yojimbo are the reason why we want our Luck stat so high. They have 130, 102 and 114 respectively. So if we need a 100 more than them, we've gotta go for 230 Luck.

You also don't need Auto-Phoenix for this fight, but I already had on my armor, and it wouldn't hurt to have it for my strategy.
Tidus, Rikku and Wakka all had armor with:
Break HP Limit
Ribbon (mandatory)
Auto-phoenix
Auto-Haste (mandatory)

I hope this helps.