Choose a good fighter such as Ken or Ryu. When you are facing the last Boss, if you are losing by one round and there is not a lot of power remaining, press Start on controller two and repeat until you win.

Use Haomaru the samurai. He looks weak and slow, but his slash and jump slash has a wide range and also does big damage. If you jump around with Airguard equipped and slash at the correct time, you will not have a problem defeating any Boss.