Though the Rekka-Ken is helpful, I suggest going back to the save with the medicines in your inventory. That was kind of my case when I fought Nobunaga for the first time (at which I didn't even know there was a Rekka-Ken until my bro told me), but it is possible to defeat him with just your regular weapons. Just try to dodge the elemental attacks as best as you can, and try for a combo with the thunder sword when he's on the ground; and use the hurricane-power thing (can't remember the name right now) while he's in the air.
However you do it, there'll no doubt come a time when the medicines will come in handy. Keep consistent with your dodging patterns and attacks, and he should be done in no time.
