I started to grind it but soon realized that I wouldn't get it that way if I couldn't come up with a consistent strategy. So I made a save right before the boss and played that part over and over again looking for a pattern and safe-spots. Experimentation with different characters eventually lead to Shishin as my favorite.
However, once I was able to beat the final boss without taking damage I continued to practice that part until I pulled it off three times in a row and then switched back to Arrange mode and got it on the second try.
Most of the time in the beginning was 'wasted' though because I didn't know about the no-bomb restriction until one of the Japanese players kindly told me, that's approximately when I started to practice from a savegame
I suggest you do the same and start by practicing only that part and once you got it down move on to playing casually.