From my experience, defeating those samurai daisho warriors requires some solid fundamentals. You'll want to start with quick strikes using light attacks but mix in those heavy hits when their guard is up, as that'll help break their posture faster. One thing that works really well is watching for their attack signals - parry the white and blue flashes, but definitely roll away from those red ones since they love chaining those together.

These guys usually hang around level 35-40 so make sure your gear is up to scratch, and something with armor piercing really helps. A good tip is staying at medium range to bait out their attacks, especially those delayed heavy swings they do when you're too far back. Their morning star throws can be nasty but using the environment for cover works wonders, and if you're playing as naoe those smoke bombs give you some breathing room.

What really makes the difference is staying patient and not getting greedy with hits. Land a couple strikes then back off, and only heal when you've got actual space. The blue combo finishers are your best friend since parrying those leaves them wide open, perfect time for a full attack string. If you're still having trouble, maybe try practicing on some weaker enemies first to get the timing down - the muscle memory really pays off against these guys.