First off, you mustn't save in between fights: You gotta do all five in a row.

Armour needs Stoneproof, Auto-Phoenix.

Weapons need Break Damage Limit and Triple Overdrive.

Use Wakka, Tidus and Rikku and set your Overdrive mode on Comrade.

Every time you get attacked, Wakka uses Attack Reels, Tidus uses Slice and Dice (tends to do more damage than Blitz Ace as well as recover quicker) and Rikku uses Hyper Mighty G to counter Wakizashi.

If Rikku gets a turn and a character currently lacks Auto Life status, have her re-cast it. If everyone's still got that, have her Quick Hit instead.

If you're strong enough, you'll easily take him down before he can use Zanmato (which comes out as soon as he overdrives). If he gets a full meter, switch Yuna in to summon your least favourite Aeon (bye bye Valefor!) to take the hit.

Dark Yojimbo is one of the easiest Dark Aeons to beat, and the best part is that you can abuse his infinity (kill the first four forms then save and return to the Airship) to get a ton of Dark Matter.