Dark Yojimbo is an optional boss battle in the International and PAL versions of Final Fantasy X.
In Yojimbo's cavern (cavern of the Stolen Fayth), go right inside, he is just after the save point. You will have to defeat him 5 times to defeat him. However you can save the game in between, but if you reset the game then you will have to start all over. He moves around the cave once you've defeated him.
 Recommended Battle Strategy
At this point you should have customized most if not all your sphere grid for a proper chance to defeat Yojimbo. He is not as hard as he seems and it only takes 4 attacks to beat him if properly powered up (Max Strength and Magic). His Luck is high so if you want to attack him you will need high luck. Wakka's attacks can do 33,333 damage to him each turn 12 times, and so after 3 overdrives he will be tired. Always remember to set the Overdrive to Comrade if you want to quickly regain your overdrive. Entrust is again the KEY to beating Yojimbo as you will need it to get 3 overdrives in quickly. After that a simple Doublecast of Ultima should do the job. Remember use overdrives later, heal your characters first. Keep Protect and Haste on your characters at all time and you need stone proof. Its best to have Auto-Haste and Auto-protect as Dark Yojimbo's Kozuko attack will also cause Full break which will need to be dispelled quickly or his next attack will do 99,999 damage (50,000 with Protect). Stoneproof is a must as when his dog attacks it will petrify your characters. There are two types of wakisashi he uses, but both are deadly, they will do around 30,000 HP damage if you don't have Protect on your characters. Heal every time he uses Wakazashi and then retaliate with Overdrives. When he is about to use his overdrive USE AN AEON TO BLOCK, it will do 99,999 damage regardless of Protect, Zanmato is the ultimate attack and can kill anything and anyone. Defeating him once is not enough you will need to do it again and again up to 5 times. Its a good idea to save and heal before going to the next fight, and also make sure that Auron is on your team when you start a fight as he will usually ambush you. You can simply just change from their. Just follow the above strategy and defeat him all 5 times, although the 5th time he seems to be a tad bit better and can do serious harm if your not prepared. Yojimbo can be very easy if you are prepared before hand and you follow the above steps. I have personally beaten him around 200 times using the above strategy (no this is not an exaggeration). If you want ribbon armor and no Dark Aeons are dropping them then just focus your energy on Dark Yojimbo as an overkill will make him drop 2 Dark Matters as a normal drop. You will need 99 of them to add ribbon to the armor, but you may also find some good armor that he drops. He will also sometimes drop Ribbon and Break HP Limit armor with 2 extra slots (which are the best ones) which should be equipped ASAP. Keep beating Yojimbo and get as many Dark Matters as you can before going on to Anima and the Magus Sisters. Penance will be impossible without the Ribbon Armours.
See Also: Dark Aeons