Hold L hard + R light + Y + A + Left Analog-stick Right then press Start during gameplay. You'll hear a sound. A cheat screen with "Invincibility", "Level Warp" options will be revealed.

Move the Right Analog-stick and the fire around the "Loading" will shift.

Hold L hard + R light + Y + A and press Left Analog-stick Right during game play. You'll hear a sound. As a precautionary: Your character's statistics will be reduced.

Complete the game at any difficulty.

Complete The Gauntlet stage to open up the game's extreme difficulty setting.