Use DSS in Battle Arena
In the Battle Arena, equip a good DSS Summon Combo (for example, Uranus + Cockatrice). Press Start, select "Items", then "Use", and use a "Mind High" (not a "Mind Restore" -- however they can work, but you will need to be extremely fast). Immediately after exiting the pause menu, press the DSS button. When your character is done casting, press Down, Forward, Up, Attack. If done fast enough, you should have pulled off a summon in the Battle Arena.

Start with All DSS Cards
Beat the game and enter 'FIREBALL' at the data screen.

Magician mode
Enter FIREBALL as a name and start a new game. If you entered the code correctly, the word Magician will appear on the option screen. All trading cards will be available under the DSS option.

Shooter mode
Enter CROSSBOW as a name and start a new game. If you entered the code correctly, the word Shooter will appear on the option screen. Nathan will have a substantial number of hearts, extra sub-weapon damage, and can use the Homing Dagger sub-weapon in this mode.

Fighter mode
Enter GRADIUS as a name and start a new game. If you entered the code correctly, the word Fighter will appear on the option screen. Nathan will be stronger and able to sustain more damage. There are no trading cards in this mode.

Thief mode
Enter DAGGER as a name and start a new game to get your character thief stats.

Get the Shining Armor
To get the Shining Armor you must beat the Battle Arena it is located in the chapel tower.