Invincibility Cheat
When you reach the title screen and see the "Press Start Button" message on the screen, press "A," "A," "A," "A," "Up," "Down," "Left" and "Right." After you hear a chime, hold the "B" button and press "Start." This should prompt another chime. Hold "C" and press "Start" again to activate invincibility. Enemies cannot hit you, but you can still fall to your death.
Select Levels
While on the title screen, press "Up," "Down," "Left" and "Right" a few times until you hear a chime. This now opens up a level select option. Press "Start" and you enter the level "The Execution Place." Hold "A" and press "Start" to access "The Floating Island." Hold "Up" and press "Start" to start at "The Village of Decay." Hold "Up" and "A" while pressing "Start" to play "Town of Fire." Hold "Down" and press "Start" to enter "Baron Rankle's Tower." Hold "Down" and "A" while pressing "Start" to visit "Horrible Faced Mountain." Hold "Left" and press "Start" to access "The Crystal Forest." Hold "Left" and "A" while pressing "Start" to play "The Ice Slopes." Hold "Right" and press "Start" to enter "Beginning of Castle." Hold "Right" and "A" while pressing start to start at "Middle of Castle." Reach the boss fight with Loki by holding diagonally down and right while pressing "Start."
Slow Motion
Use the same title screen code as the invincibility cheat -- "A," "A," "A," "A," "Up," "Down," "Left" and "Right" and you will hear the chime. Begin the game. Pause the game and hold "B" as you press "Start" to unpause the game. This will make the gameplay occur in slow motion until you enter this combination again to turn it off.
Using Magic
Maximize your weapons by obtaining magic armor. This will cause a red gauge to appear on the screen. Hold down the attack button to fill the meter. It will turn gold when ready. Release the button to unleash a devastating magic attack on foes. Be careful as one hit will cause you to lose your magic armor.