Play as a Monster
From the main menu, enter VS MODE. Move the cursor to 1P VS COM, and press Ls+left+A. Select the monster you want to play. At the battle, you should be able to play the monster against a computer-controlled Ultraman.
Play as Baltan II
Enter the "Play as a Monster" code, then move the cursor to Baltan and press Ls+Up+A. You'll now be playing Powered Baltan (Baltan II) against a computer-controlled Ultraman.