In Mission 3, you can find it by shooting the corner of a specific block in the first secret cave area.
When active, your character (Walter or Tyra) will die in a single hit . Metal Slug Advance
While some are just for display, many cards grant gameplay bonuses like increased weapon power, new "Slug" modifications (like the Slug Gunner ), and access to hidden map routes. In Mission 3, you can find it by