While playing in Story Mode, you can get to the arena in stages 3, 5 and 12. To do so, kill all the enemies on the level (including the hidden plants and disfunctional robot on stage 3). When you've done this, jump to the back of the screen (the third row) and walk past the sign. You'll arrive in the arena where you can fight as many battles as you like to build up your stafs. have fun!