According to one of my friends who used Norm's Flutes to complete the Boss Monster areas quickly, he's already reached and completed Area #10 of Martial Arts Hall. According to him, the fifth and final Boss Fight is against Sueki Suezo! I did the math and I should be able to finish Area 10 myself (without Norm's Flutes) in 27 hours; that's 3:30PM EDT tomorrow.
I'll be sure to get more Screenshots and add them into Sueki Suezo's entry. Finally we'll be able to see what it looks like.
I'm already planning how to spam Areas #2, #4, and #10 with the three currently unobtained Rares in hopes of discovering if we can get one. If it is possible, you can rest assured I'll be making sure that all 3 are obtained and that there is access available to them.