Play Metal Slug 3 or Metal Slug 5 on the Hard difficulty setting. Immediately after you defeat the final Boss, save the game, then quit to the main menu once the credits begin. You should get five Tokens. Then, load the saved game file and repeat as desired.
Unlock the following art galleries by completing the corresponding games:
ART GALLERY - GAME Art Gallery 1 - Metal Slug Art Gallery 2 - Metal Slug 2 Art Gallery 3 - Metal Slug X Art Gallery 4 - Metal Slug 3 Art Gallery 5 - Metal Slug 4 Art Gallery 6 - Metal Slug 5 Art Gallery 7 - Metal Slug 6 Tonko Gallery - All seven Metal Slug games on any difficulty setting with limited continues.