Yeah, my main frustration starting out was navigating the menu so that I could use items to heal and stay alive long enough to get comfortable with the combat.
Once I understood (with everyone's help here) how holding L activates the item bar and Y and A scroll left and right, and how you customize the radial through a separate menu, I was able to keep my character alive with potions, mega potions, max potions, first aid kits, etc.
The Great Sword was challenging at first, since it's slow and locks you into some long animations, but it taught me to mind my spacing and pick my shots: Dash in, unsheath, charge and hit (and maybe hit again for a combo), then roll to the side, sheath and run away while drinking a potion. At that point it's just a matter of deciding when to attack - usually after Mizutsune is done twirling around blowing soap bubbles. It was fun, him leading me to six locations, each with different terrain and wildlife. He beat me three times, but then I got him.
And now, I'm trying the Long Sword and Sword & Shield, and wow -- these are MUCH easier. But I also appreciate the high-impact blows of the Great Sword. So I'm glad I tried it first, and that I completed quests with it, although I think I would recommend people start with a faster weapon -- it may have gone more smoothly. :)