Here's a quite painless way to play as the hidden Boss, Zankaro! To select him, choose Vs. Mode and go to the Character Select Screen. Hold down Start and highlight the following characters in order:

Start on Haohmaru and then go to Genjuro, Basara, Kyoshiro, Ukyo, Rimruru, Haohmaru, Shizmaru, Nakoruru, Hanzo, Amakusa, Gaira, Galford and then Shizmaru. Keep holding Start and then X+Circle.

Zankuro should then appear as a selectable character.