Pause the game and press L+A+B+Y unpause it. Your ammo becomes unlimited for every weapon.
Complete missions 1, 2, and 3 to unlock missions 4, 5, and 6. Each of the new missions are based on the previous 3 missions with higher difficulty: Mission 4 is based on mission 1 with a single life, double damage weapons, and no continues. Mission 5 is based on mission 2 with a time limit in each room and no continues. Mission 6 is based on mission 3, with very little health, few health power-ups, no other power-ups, and no continues.
Collect all illustrations in the game to unlock the original Bruno from Die Hard Arcade.
At the character selection screen, highlight Ivy, then hold Start to unlock Cindy from Die Hard Arcade.
Successfully complete missions 4, 5, and 6 to unlock the Monkey.
Successfully complete the game once.
Successfully complete missions 1, 2, and 3 without using any continues to have infinite credits for the Tranquilizer Gun mini-game.