Defeat the final boss. Just as the boss is destroyed (while it is exploding/going up in flames), .
There is one primary button-based code commonly cited for accessing a hidden menu within the options screen. Metal Slug Anthology Psp Cheat Codes
Metal Slug Anthology on the PSP, there is no traditional "Cheat" menu built into the game's start screen like in some modern titles. Instead, unlocking the full experience involves a mix of button-sequence codes glitch-based farming emulator-specific hacks for those playing via PPSSPP. The Hidden Secret Menu Defeat the final boss