BOSS ANALYSIS : Little that you wouldn’t expect except (surprise!) additional effects added to moves. Wakizashi - his most-used move for me - is a simple physical to all; it still hits hard at 244 Strength, though. For Daigoro, you’ll see Poison and Petrification alongside single-target non-reducable physical damage. Kozuka is blockable and physical and single-target, and it causes all the Breaks and Slow. Finally, there’s the Overdrive, Zanmato - this is an instant-KO to everyone with an Auto-Life removal, the KO is regardless of Deathproof or Ribbon, meaning the only way to survive is Aeon cannon fodder.
MAIN STRATEGY : Even though Ribbon doesn’t prevent Zanmato, you’ll want it to help stop Daigoro dead. Auto-Haste is also nice to deal with Kozuka, and to prevent the Haste from being Dispelled when you get rid of the Breaks. Auto-Protect is less usable (only good for Wakizashi/Kozuka), but it can be nice to have nonetheless. With that buffing, you ought to be getting hit for under 10,000 HP when not Broken, so Auto-X-Potion also works well.
There’s not a lot to open the battle with; instill Super Mighty G if you don’t have Haste AND Protect on yet (Mighty G for just Protect, Hastega for just Haste), and that might as well suffice for the buffing. As usual, Ultra NulAll’s stat boosts can also be nice to lower the damage from Dark Yojimbo further if need be.
After that, your offense is pretty simple - Quick Hits, due to the faster recovery rate, further aided by Auto-Haste. A magical offensive is another way to go, since magic is about 46% stronger than physicals on Yojimbo, in which case you can simply Doublecast Flare/Ultima instead, whichever you prefer, really!
After the fight, you will have to chase after the summmoner in the cave, and on foot into another room. Since you have to fight Dark Yojimbo five times total, having some back-up allies from the start to induce Overdrives with (mostly those opening Mixes) via Entrust can help a lot.