Choose any difficulty and any path. Defeat the first form of the final boss with at least one continue left to unlock the good ending, even if you drop down to your last continue on the second form.
To view the secret message, go to the title screen and enter the following code.
In the options press Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, A, B, C, A, B, C, A, B, C, A, B, C, A, B, C.The mnemonic is: UDLR 5 times then ABC 5 times.