To do it is simple: On the character selection screen, press C six times, then B three times, then, again C four times, all in the time of less than three seconds. It's kinda tricky, cause you need to do it really fast. Once you have done it right, two arrows will be shown on the two sides of the screen, which is where the boss characters are. Have fun.

Remember: C = kick button, B = strong slash button

It goes like: C x6, B x3, C x4, all in less than 3 seconds.