well, no one ever had or will have trouble beating the dark aeons in FFX-2
but anywho,
If you charge up your overdrives for everyone before the 1st battle then you will stand a good chance.
To do this quickly, equip everyone with their celestial weapons and set their overdrive mode onto "Victor" and win two battles to fill an empty gauge up.
After that change all character's overdrive mode to "Comrade" and make sure EVERYONE can use "Entrust" ability
Now, Give three characters a 1st strike weapon, excluding Yuna, Wakka, and someone of your choice who has a 1MP skill in their weapon. Yuna and Wakka must have their celestial weapons for this. You need to cast "Auto-Life" white magic so two of your non-Wakka characters need 1MP cost
Let's say you gave Kimari, Rikku, and Auron a 1st strike weapon. All you need to do is put them up the top of the list so they start the first battle as the first one is almost always an ambush.
Start the battle and switch the first person who gets to strike to Wakka. Imediately use Attack Reels and deal 12 lots of 20000 or so damage.
Next use the second person to entrust their overdrive back to Wakka (or switch them to a stronger character if they arent' strong enough)
Finally Switch the third person out for Yuna and cast "Auto-Life" white magic on anyone you want to.
From there the first battle will go something like this:
Yojimbo strikes
You cast Auto-Life
You use Attack Reels
You use Entrust
After 3 Attack Reels it will take about 6 normal hits with cahracters who have 255 strength, 255 accuracy and their celestial weapons. Alternativly you COULD use Attack Reels a 4th time, but it's proabbyl better to save that for the next battle
Once you beat him once, you get to fight him 4 more times. so use Curaga outside of battle with someone who you don't use to cast Auto-Life (since we don't care about them and their MP)
Engage with Dark Yojimbo and keep up the Attack Reels, Entrust, and Auto-Life
Since you have 1MP cost you will never reach zero MP, and since you have your overdrive set to Comrade you will quickly gain your Overdrives
it may pay to have two of your active caharacters with Auto-Protect if your HP isn't high
The I is moving 
I would rather live in my own shadow...at least then I would be compared to a past I'm more familiar with ~I~
Fear me, Hate me, Curse me, for I will question your every move ~I~
My past hold much pain and loss. I will conquer it by creating the perfect future ~Ixidor the Reality Sculptor (Magic the Gathering)~