Knife Edge: Nose Gunner

Stage select
Choose "Team " or "Story" mode. When the level number is displayed enter
the following button combinations:
Hold C-Left + C-Right + C-Up + C-Down + R and press Right, Up, Left, Down

Unlock "Super Hard" difficulty level:
Hold L + R + C-Up and press C-Right, C-Left, B at the Kemco logo.